In a large soup pot, melt the butter over low heat. Add the sliced onions and sauté slowly for 1 1/2 hours, stirring frequently, until they become golden brown. This slow cooking develops the onions' natural sweetness and depth of flavor.

Incorporate the freshly ground black pepper, paprika, and bay leaf into the sautéed onions. Stir well to combine. Sprinkle the flour over the onion mixture and cook over low heat for an additional 10 minutes, stirring constantly to prevent sticking. This step helps to thicken the soup.

Gradually add the beef bouillon to the pot, stirring continuously to ensure a smooth consistency. If using white wine, add it at this stage.

Bring the mixture to a gentle simmer and cook for 2 hours, allowing the flavors to meld together beautifully. Adjust the color to a rich brown, if desired, using caramel coloring or liquid brown sauce. Season with salt to taste.

For optimal flavor, refrigerate the soup overnight. This resting period enhances the depth and richness of the soup.

When ready to serve, reheat the soup gently. Ladle approximately 1 cup of hot soup into each ovenproof bowl. Top with three 1/2-inch slices of French bread, ensuring they cover the surface. Generously sprinkle grated Swiss cheese over the bread.

Place the bowls under a preheated broiler for about 5 minutes at 550 degrees Fahrenheit, or until the cheese melts and achieves a golden-brown crust. Carefully remove from the broiler and serve immediately, exercising caution as the bowls will be hot.