In 3-4 quart crockpot, combine onions, melted butter, olive oil, and sugar. Cover crockpot and cook onions on high heat for 30-40 minutes or until onions begin to brown around the edges. Stir thoroughly.
Sprinkle flour and pepper over the onions; mix well. Let cook for 15 minutes longer on high. Then add beef broth and stir again.
Cover crockpot and cook on low for 7-9 hours until onions are tender.
I prefer french onion soup in the crock pot over the usual stovetop method because the onion flavor is richer because of the long cooking time. Tastes just like the soup from a top restaurant. One note though, I don't use shredded swiss, I use thin slices overlapping slightly on the french bread rounds.