Heat the oil and butter in a large skillet over low heat. Add the onions and pepper and cook for 30-40 minutes or until the onions are very soft and sweet.
Add the vinegar, stir and cook for 15 minutes more. Cool, cover and refrigerate for up to 3 days.
Recipe by Culinary Thymes at https://www.culinarythymes.com/red-onion-marmalade/