In a large skillet over low heat, melt the butter with the olive oil; stir in the brown sugar and add the onions. Cook the onions over low heat, stirring often until golden and caramelized, approx. 50-60 minutes.
Combine sliced onions, butter, salt and brown sugar. Stir, cover and cook on low heat for 8-10 hours or until golden and caramelized.
If desired, you can add sweet green or red bell pepper slices to the caramelized onions, as they cook.