Clean and chop the onions. Mix the oil with the butter and heat them, preferably in a container with a thick bottom. Add the chopped onions and powdered sugar, fry for 20 min. while stirring nonstop. Once onions are caramelized, add the finely chopped garlic and braise for 1 min.
Pour white wine and the broth over the onions, season with salt and black pepper, saute on low heat about 60 min.
Serve the soup with bread slices toasted with feta cheese or cheese - place them on the bottom of the serving bowls and pour the hot soup on top.