When travelling at a campsite, our resident cook made a Bolognese, and for fun I asked the Italian campers what they thought about it (playing on the stereotype that they are serious about their food) not expecting them to actually be serious about their food! They looked at me kind of disappointed and told me there was garlic in the Bolognese… "There shouldn't be garlic in a Bolognese" they uttered quietly.
Since then this question has driven me crazy. I want to know technically what garlic and onion is adding to the flavour, why and when it is desirable, and when it isn't.
Best Answer
The general category for vegetables/herbs (that are not leafy green herbs, or spices) added to a dish for flavor (rather than bulk/texture/nutrition) is "aromatics", and that describes what they are there for: Add some basic aroma to complete the flavor profile. Typically added early and sauteed or sweated with some kind of culinary fat present to bind the aroma to something that can carry it.
Commonly used: Alliums (onions, spring onions, leeks, garlic,....), celery, rhizomes (ginger, galangal, (finely cut) carrot ...), capsicums (chile peppers, (finely cut) bell peppers..) ... Arguably, one might include tomato concentrate given the way it is sometimes used.
Sometimes the aroma is used as is, sometimes it's augmented by treating the aromatics in a way that adds Maillard reaction products.
There is also some effect on taste, mostly sweet and bitter components.
Some categories where onion and garlic are not always suitable:
Sweet dishes - there typically is no sauteing step involved, and vegetable flavors typically do not work too well here - also because there is too little salt to offset bitter elements introduced.
Dishes made in any culinary tradition that does not approve of alliums, e.g. Jain food, or recipes where alternatives like asafoetida are supposed to take this role completely
Food to be served to people who want to avoid smelling of garlic or onions, or who are allergic or intolerant to it
Raw preparations, sometimes - raw onions and garlic are very spicy and pungent, and have an even higher risk of causing undesirable breath seasoning.
Anything where the garlic would burn (though, of course, you can add a garlic preparation afterwards) - burnt garlic is a generally very undesirable flavor.
Using "normal" onions in dishes calling for shallots and/or spring onions can have undesirable flavor and/or texture effects.