Garlic, onions, shallots and other aromatic plants of the alium family are deeply flavourful ingredients that are indispensible in most kitchens. A whole onion, shallot or clove of garlic won't have much - if any - odour (unless it's rotten; then it will smell awful), but when you slice into any of them, a sharp scent will be released. How strong the scent is - and how much flavour is given - depends on how a specimen has been prepared. If you use a very sharp knife to slice it (a dull knife bruises the ingredients' cells, rather than opening them cleanly), fewer of the tear-inducing sulphurous compounds will be released. If you purée an onion or clove of garlic, or chop it into very fine pieces, the flavours will be much stronger than if it's cut into large-ish chunks. Rinsing chopped or sliced aliums with cold water washes away the sulphur compounds, making the flavour milder.
You can also vary the flavour of aliums by using different cooking methods. Cooking them over a low heat for a long time turns them from crisp and juicy, with a strong, pungent flavour, to soft, muted and sweet. Conversely, if you cook them over a high flame, they can become acrid and bitter. This happens in an instant: one second, the onion or garlic is golden brown (when it has a pleasant flavour), and the next, it's burnt; so they need careful attention.
Although it doesn't happen often, garlic oil, made by soaking raw chopped garlic in olive oil, can be deadly because of the potential for botulism - the spores of which grow in soil and thrive in an anaerobic environment (such as being covered in oil). To be perfectly safe, raw garlic oil should be made the day it's required and any leftovers discarded.
Truc (tryk): noun, masculine, trick, gimmick, device. A French word for a chef's secret.