Not something I use directly in my practice but onions, leeks and garlic all form a significant part of my diet. There is some evidence that garlic can enhance the immune system, interestingly at least one source links this to compounds formed when it is heated meaning that if this is the only mechanism, those who (like myself) sometimes take it raw for this reason will not get the benefit. However as I take it cooked with other food most days, perhaps I gain anyway and am just wasting my time taking it raw?
Onions and leeks are seen by many as cleansing foods though as with all foods it is finding the right balance for our particular needs that is important. Generally speaking, no food type is bad unless one has a peanut or other allergy.
While my reading and nursing training gives me some insight into nutritional needs, I am not an expert in this area so I can pass on information along with sources for it but if a client asks me I do tell them that I am not qualified to give dietary advice. I do however know a number of nutritional therapists who I can give details of to clients should they wish it.
While garlic does have an essential oil it is not widely used in aromatherapy massage for what I hope are obvious reasons! It is sometimes used in preparations for ears and also for expelling worms. (We give garlic to our chickens once a week and to date have seen no evidence of any of them getting worms.)
Finally, there is no evidence to support the view once put out by the Catholic Church that banned it due to it’s inflaming passions.