Melt the butter in a heavy 4 quart sauce pan. Add onions and sauté over low heat until soft. Sprinkle in flour and stir. Pour in the stock. Season with salt and pepper. Cover and cook over low heat for 45 minutes. Stir occasionally.
Divide soup into 8 crock type bowls. Place 2 slices of toasted bread on top of each bowl.
Mix cheeses together in bowl. Sprinkle soup with cheeses. Place bowls under broiler until cheese is melted and browned. Serve immediately with chilled Italian Pinot Grigio.