In a large bottomed pan, melt the butter over medium heat. Add in the sliced onions and reduce the heat to low. Stir until the onions are well coated in the melted butter.
Continue to cook the onions, stirring every five minutes or so until they are a deep golden brown but still have an al dente crunch. About 45 minutes.
Use the broth or acid to deglaze the pan, using a wooden spoon or other utensil to scrape up any stuck-on bits of caramelization. Add more liquid if necessary until as much as possible is released. Stir it into the caramelized onions. Salt and pepper, to taste.
*You can use whatever type of broth or acid you would like. I recommend any of the following: wine, apple cider vinegar, lemon juice, balsamic vinegar, red wine vinegar, orange juice, etc...