Melt 1 tablespoon of the butter in large stockpot or Dutch oven on medium-low heat. Add 1/3 of the onions; cook and stir on medium-high heat until golden brown. Repeat with remaining butter and onions, cooking in batches.
Stir in stock, wine, bay leaves, garlic powder, thyme and pepper. Cook on medium-high heat 20 minutes, stirring occasionally. Remove bay leaves before serving.
Meanwhile, top bread with cheese. Toast bread in broiler, oven or toaster oven until cheese is melted and bubbly. To serve, ladle soup into 4 bowls and top each with a toasted bread.