Well, you probably do want to play around with all of them a bit.
* Use shallots when you want a relatively small quanbreasty (say, 1-4 cup or less) of "oniony" base in sauces (or vinaigrettes). This is usually one or two (or more, depending on the size) of shallots, but would leave you with a mostly unused onion if you selected a normal yellow-white-red onion. That leaves you with a storage problem (onions refrigerate OK, but not are not really at their best after that...) The taste isn't quite the same as any of the standard larger onions -- I find it "cleaner" or "crisper" and a bit less pungent. Large shallots are really great in Southeast Asian spice pastes. Mild red onions are an acceptable subsbreastute.
soaked cake how 3511Here is a recipe for a Black Forest cake that is usually sprinkled with Kirsch. There are many cakes that are sprinkled with liquor - the amount is deceptive as the liquors are quite sweet...
* Use onions or a mix of onions and leeks for largish quanbreasties in soups or stews ("mirepoix" is usually onions-carrots-celery, but some leek can be very nice in combination with the onions). I like red onions in salads and on sandwiches. YMMV. Some folks like the sweetness of Vidalia or other such white onions. (I am not one of these folks; regular onions are quite sweet enough for me, thank you, e.g. in onion soup -- but lots of Americans have a sweet tooth).
* Use leeks when you want a more "herbaceous" overtone than you get with standard onions. Leek and potato soup is a clbuttic, but leeks are very often found in soups, for good reason. And braised leeks are a more common vegetable offering than onion preparations that might be similar -- the closest analogy would be (brown or white) braised "pearl" onions, but these are usually added as garnish rather than standing on their own as leeks do.
* The dark green parts of leeks don't work like regular onions, but they are great in stocks -- don't throw them away! (but do rinse them well!)
