Place the butter in a saucepan on medium heat.
When the butter starts foaming add the onion. Sauté until the onion is soft and translucent, and lightly browned, about 5 minutes.
Sauté until the onion is soft and translucent, and lightly browned, about 5 minutes.
Add the stock. Stir in the garlic, salt and pepper.
Boil the soup for about 30 minutes.
Preheat oven 400 F (200 C.) Place a few slices of bread in the single-serving oven-proof bowls.
Fill about half way the bowl with the soup. Distribute evenly the onion and the liquid.
Spread evenly the cheese on top of the soup bowls.
Place the bowls on a cookie pan. Bake in the oven for approximately 30 to 40 minutes until the cheese is melted and browned all around. Serve hot at once.