1. Preheat oven to hot (400F)
2. Saute the onions in four tablespoons of the butter until golden, stirring often.
3. Add the stock and boil 10 minutes. Season to taste with salt and pepper.
4. Place the toast in one large or four small casseroles, add the soup and sprinkle with cheese. Dot the top with the remaining butter
5. Bake until the top is golden brown.