Heat butter and oil in large pot
Add onions, cook on low-medium heat for 30 minutes, stirring every few minutes until onions are brown.
While the onions cook, place cheese on the bread slices, then place on a baking sheet. Broil on medium until cheese begins to bubble.
Add garlic and sugar to the onions in the middle of the cooking time.
Sprinkle flour on the onion mixture, stir to thicken. Add beef broth slowly. Stir over low heat until soup is thickened.
Add thyme and pepper, then simmer for 25 minutes. Remove from heat and add white wine.
Place a slice of bread with cheese in the bottom of bowls and pour the hot soup over it.