To make caramelized onions, saute sliced onions in 1 to 2 tablespoons of olive oil or butter (or 1 tablespoon of each) and 1 teaspoon of sugar per onion. Cook and stir occasionally over low heat for 15-20 minutes or until onions are golden brown and tender.
You can slice a bunch of onions at once. Cut onions into uniform shapes and thicknesses to ensure they will cook evenly. Onions shrink as they cook, losing about 2/3 of their volume as the water evaporates, so slice more than you think you'll need.
Onions can be caramelized a day in advance. Cool, cover and refrigerate. The next day, reheat and serve.