Heat olive oil and butter in a cast iron skillet over medium heat until oil covers the pan evenly and is hot. Add the sliced onions and stir enough to coat the onions with the oil and butter.
Continue to cook onions over medium heat, stirring occasionally until onions begin to brown, about 10 minutes.
Season with 1/2 teaspoon of salt and unrefined sugar, spread onions evenly over bottom of skillet, reduce heat to low.
Allow onions to gently caramelize over the next hour, stirring occasionally to ensure they caramelize evenly. Onions should be deep, golden brown when done.
When using Vidalia, Maui, or other super sweet onions, omit any sweeteners, as they are not necessary. There is no exact time when the magic happens, some let onions caramelize for up to 2 hours. Caramelized onions can be stored in the refrigerator for up to 1 week or frozen until ready to use.