If I make a meatloaf, I will very carefully cook the onions and celery and bell pepper in a skillet with a little oil, adding a bit more oil if needed. I don’t really caramelize the veggies, I just get them tender.
I’ll also do this with soups and stews, though I usually don’t use bell peppers in soups or stews, just the onion and celery.
I roast chickens in a big Dutch oven, I peel and slice potatoes and lay down a layer or two. Then I peel onions and carrots and chop them into quarters, and cut a rib of celery into quarters. One quarter of each vegetable goes into the neck cavity, half of the veggies go in the body cavity, and the rest are scattered around the bird. I don’t precook the onions in this case. If I want stuffing, it’s cooked outside the bird, possibly with some drippings in it. The vegetables flavor the bird very nicely, and are delicious.
Potato and leek soup is one of the most comforting soups there is on a cool or cold day. I usually put some dried parsley on top of each bowl. This soup is good with just the potatoes and leeks, or with bacon or ham in it, too.
I grill sliced onions and mushrooms in some butter, put them in a heatproof dish and put them in a slow oven, and then cook a steak or chop or chicken breast in the same skillet, then serve the mushrooms and onions on the side of the chunk of flesh. It makes the meat look more substantial. Sometimes I cook chopped celery with this, too.
Onions can add a lot of flavor for a very low cost, and they’re good for you. The butter isn’t so good for you, which is why I try to use just a bit of it.
Red onions are usually served raw, and are commonly used on burgers and pizzas.