When sweet onions are on special I set aside half a day to make caramelized onions because they add an incredible amount of flavor to any dish. Most recipes call for several minutes of cooking. I take at least four hours.
The heat is medium-low. Listen for cooking sounds. If you hear anything above just a whisper, the heat is too high. Stir every quarter hour or so.
That's it. Refrigerate and use in anything that calls for onions. If you feel ambitious, proceed on to make some Sweet Onion Marmalade. Once you've tried caramelizing your onions this slowly, you'll never go back to high heat.