Caramelized onions are a remarkably versatile and easy to make with the simplest of components - onions, oil, salt and heat - but I find them underutilized. Basically, all you have to do is cut up some onions, and cool them until the sugars burn a little. That's about it. The caramelization process is technically known as the Maillard reaction, and can be accelerated by increasing the pH of the recipe. The most common way this is done in the kitchen is the addition of a little bit of sodium bicarbonate AKA baking soda (not baking powder, that's a discussion for another day).
The folly of doing this, however, is if you are using red onions, which contain the pH reactive anthocyanin pigments. They'll turn a different color that may be a tad alarming.
Pictured above is a calabasita and caramelized onion quiche, prepared by putting down a layer of caramelized onions in a frozen pie crust, then topped with calabasita squash chunks sauteed in olive oil and cayenne pepper, and finally a custard of eggs and cream poured over everything before being baked. I suppose cheese would match this as well, but I was out of cheese.