Onions have a lot of water in them. Take away all the water and you are left with amazing flavor and sweetness. Of course the process to do this takes some practice and a lot of time.
Trust me, it is worth it.
Red onions will give the best result when they cook down, but any type can be caramelized. Make sure to slice the onion as thinly as possible. I find cutting it in half and then slicing makes it easier at the end, but you can slice it whole as well.
Melt two tablespoons butter and two tablespoons olive oil for each onion in a large pan over medium heat. The oil will keep the butter from scorching, while the butter will make everything taste better.
Add onion to melted fat breaking it apart as much as possible. Mix well coating the slices in fat. Turn heat to a very low simmer.
Stir the onions every five minutes or so to keep them from sticking.
Keep stirring and pressing the onions into an even layer over the bottom of the pan. The picture above is 20 minutes into cooking.
Keep stirring and spreading the onions until they are a deep dark reddish brown. The picture above is about 45 minutes in and the onions still need about 10 minutes.
As you can see the finished product is quite small, but the flavor contained in each auburn thread is enormous. The onions can be refrigerated for at least two weeks safely.