Does it matter if I sauté onions for high liquid foods? Many (most?) recipes ask that you sauté onions before adding other ingredients when making soups, stews, or other dishes that contain lots of liquid (as opposed to something like a stir fry).
I'm not taking about caramelization -- I understand the purpose of that.  I mean recipes that ask you to &quot;sweat&quot; the onions or cook them until they're just translucent, with no browning.
Cooking the onions this way makes sense if you want to reduce the moisture and concentrate the flavors.  But it seems to me that if you're adding the onions to soup, for example, you're reducing the moisture just to put moisture back in it.
Does it really make a difference in a soup or stew, as opposed to simply adding the raw onion along with other ingredients?
It makes a big difference. The heat you can impart to anything by boiling is limited to 100°C (212°F). This is too low for some of the flavor magic to happen, like maillard reactions, which start about 140°C. A pan's temperature can get much higher than boiling water, which is why you saute them first. If you try to make onion soup without sauteing them first they'll soften, but they'll never lose that raw onion flavor, and you won't get the sweetness or depth of flavor from the maillard reactions.