Why onion and garlic are cursed in Hinduism?

Onion and Garlic are considered as Tamasic foods producing sedative effects on the mind and the body. According to Hindu religion, these foods should be avoided as they can cause mental dullness and physical numbness due release of heat of such foods on our circulation system. Moreover, Tamsic foods promote death, destruction, ignorance, dissolution, sloth and resistance. This version was told by Lord Krisna in a preaching session to his disciple Arjuna.
Both the items Onion and Garlic are considered as Tamsic or aphrodisiacs and its consumption will elevate libido to the consumers and such a habit of consumption of such foods will alleviate their calmness and sharp thinking of the complex matters.
There is a belief that onion and garlic negativily impact the ability to cognitive thinking. Raw onions especially if consumed in a medium to large quantity are thought to have an impact on our rational thinking since both could alter your temperament because of release of excess heat on the body system.

Onion and garlic are tamasic food which are considered detrimental to mind and body. They affect the reasoning power of brain and make the body dull and lazy. So it's advised that people should refrain from consuming onion and garlic. In religious scriptures like Gita, Manusmiriti people are strictly stopped from eating garlic and onion.

"Foods that are too bitter, too sour, salty, hot, pungent, dry and burning are dear to those in the mode of passion. Such foods cause distress, misery and disease." (Bhagwat Gita 17:9)

tamas tv ajñana-ja viddhi mohana sarva-dehinam pramadalasya-nidrabhis tan nibadhnati bharata (Bhagvad Gita 14:8)

"O Arjun, tamo guna, which is born of ignorance, is the cause of illusion for the embodied souls. It deludes all living beings through negligence, laziness, and sleep."

krodhad bhavati sammoha sammohat smiti-vibhrama
smiti-bhranshad buddhi-nasho buddhi-nashat praashyati ( Bhagwat Gita 2:63)

"Anger leads to clouding of judgment, which results in bewilderment of the memory. When the memory is bewildered, the intellect gets destroyed; and when the intellect is destroyed, one is ruined".

In Manusmirti 5:5 it' stated:

lasuna gñjana caiva palau kavakani ca 
abhakyai dvijatinamamedhyaprabhavani ca

"Garlic, leeks and onions, mushrooms and all that proceeds from impure things are unfit to re eaten by twice-born men."

 Again on 5:19 eating garlic and onion etc is a reprehensible act.

chatraka vivaraha ca lasuna gramakukkuam 
palau gñjana caiva matya jagdhva pated dvija

"The mushroom, the village-pig, garlic, the village-cock, onions and leeks,—the twice-born man eating these intentionally would become an outcast".

• Tamo guna is the opposite of sattva guna. It causes doze, idleness, inebriety, chaos, and gambling. Those who often consume are deprived of distinguish what is right and what is wrong, moreover, they begin to behave in infra-dig way. They lose interest in fulfilling their duty as they feel lethargic. They keep on falling into the pit of ignorance. Their ignorance entail them to ill manners

• When the sharpness of intelligence is hazed, it steers to the confusion into the memory and it results in the harm the ability of understanding. As a matter of fact the intellect is the innermost source of guidance when it gets destroyed, he is ruined. In this manner, the path of descency and divinity is blurred and impiety, ignorance and irreligiousness prevail.

• Since these things are prohibited and highly disliked in scriptures, and the person who consumes them commit reprehensible act. Therefore, according to scriptures he should be out cast as the punishment for this consumption until he abstains himself from taking them.

P.S: Thanks to my friend Mr Saghir Ahmad alias Raju who is a scholar of these scriptures helped me.

1, Our basic health sastra is Ayurveda. According to Ayurveda foods are divided into three categories - sattvic, rajasic and tamasic - foods. Onions and garlic are classified as rajasic and tamasic. They are supposed to increase passion and ignorance.
2,They stimulate the central nervous system. Garlic is a natural aphrodisiac.
3. Ayurveda believes onions are harmful to the lungs, garlic to the heart.
4. Ayurveda says that they are harmful physically, emotionally, mentally and spiritually.
5. Garlic is toxic to humans because its sulphone hydroxyl ions which are poisonous to brain cells.
6. Homeopathy says that red onion produces a dry cough, watery eyes, sneezing, runny nose.
These are some reasons for rejecting these items by many Hindus.

According to Ayurveda shastra, onion and garlic belongs to Tamsik food. It is advised in Mahabharata that tamsik food has a negative impact on the human mind and body. Onion and garlic both are impure food. As per the Hindu religion, People should avoid this food as it makes our body dull and lazy.
Apart from this, onion and garlic are not healthy for our body as onion affects our lungs and garlic affects our heart. Both can change the temperament of our body. It is believed that garlic release toxins which are poisonous for our brain. As per Bhagavata Gita, This kind of food causes distress and disease in our body.
In Homeopathy, consuming onion and garlic are strictly prohibited. If you have this item then the effect of medicine would be zero. Some people avoid this due to the sharp smell.

According to our scriptures and Ayurveda, there are many foods and spices which are under the taamsik category. Though no scientific reasoning is there to do so but this had been told by the ancient scholars and must have been based on their experience. As we believe many things mentioned and directed in our culture, mythology, scriptures, Ayurveda, and other traditional wisdom matters, so many of us avoid these food items in our life. One interesting thing about these spicy, pungent, and specially flavoured items is that they are very useful and effective in preparing tasty and delicious cuisines and this is the reason why they are being consumed by the people in such a large quantity. There are many households where preparing food without onion and garlic is unthinkable.

New generation is not very much concerned about these beliefs and take these items as a routine way in their food. Hotels and restaurants also use them in almost all their dishes. Modern medical persons from any discipline even advocate use of garlic for so many things. Even onion is supposed to have many health benefits. So we have to take the ancient knowledge also sometimes with a pinch of salt because we have not validated those things scientifically.

There is no curse or ban in Hinduism for Garlic and Onion. Many people avoid them due to
a) The thought of damasa food which makes us dull and unable to work freely after eating.
b) Number two it induces the sexual interest in one it is avoided by the people especially in those days they want to stay freely.
c) With a view to follow etiquette that is to avoid the smell while talking to seniors and respectful persons, and to avoid embarrassing situation as it releases gas from body.
My uncle often avoid onion whenever he is going to meeting with senior people etc.,
I read in one book that the famous Cine Actor Sivaji Ganesan mostly avoid onion during Cine shooting close up scenes to avoid the inconvenience to other artists.

Onion and Garlic are coming under the category of 'Taamasa aahaar'. Food items are also categorised into three categories or Gunas - Satva, Rajasa and Taamasa. Taamasa denotes dark and hence believed to increase negative characteristics in those who consume it. They are believed to increase the carnal desires. Hence they are avoided on days of devotion and days of religious significance. Those who are following Satvik life style totally avoid these two items.

Practically also, the two items Onion and Garlic, have pungent odour and even the body and breath odour of those who consume these regularly can be pungent. For that reason also many people avoid these two items.

However as both have some medicinal and beneficial qualities, they are not made fully taboo, but allowed with regulated use. Garlic, if consumed in more quantity, can increase body heat and heartburn. So even though certain people totally avoid them, many others consume Onion frequently; but restrict Garlic to few days. There are others who totally avoid both Onion and Garlic on special days like Ekadasi, Amavasya, ancestors' death anniversary days etc.