Slice the onions to an even thickness (you can decide how thick, just keep it consistent so that they cook evenly). Add the sliced onions and butter to a large pot. Season with a bit of salt. Cover and cook on medium heat until the onions are soft and translucent, about 15 minutes, stirring once or twice.

Remove the lid of the pot and cook over medium high heat, stirring occasionally as the onions begin to take on color. Once turning pale gold, reduce the heat to medium begin to stir more often to prevent catching and burning. As the color deepens and the onions begin to stick more, add a bit of water and stir to scrape all the browned bits off the bottom of the pan. The more you cook the onions, the sweeter they will become and the more caramel flavor they will take on. The entire process should take from 1-2 hours. Be sure to stir often towards the end of cooking. Once the onions have reached a caramelization level you like, remove from heat and let cool.

I like to store cooled onions in a couple of ziplock bags. Spread the onion thin and flatten the bag. Place onto a baking sheet and freeze until solid. This way you can break off bits of caramelized onion as desired and the flat bags stack easily in the freezer. They will keep for several months stored this way.