Peel and thinly slice the onions.

In a large pot or Dutch oven, add the beef stock.

Add the sliced onions to the beef stock.

Bring the mixture to a simmer over medium heat.

Continue to simmer the onions in the beef stock, stirring occasionally, until the onions are very tender and caramelized (about 45 minutes).

Season with salt and pepper to taste.

Preheat your oven's broiler.

Toast the sour dough bread slices until they are dried out and hard, creating croutons.

Pour the hot soup into oven-safe bowls.

Place a crouton on top of each bowl of soup.

Cover the crouton with a slice of Mozzarella cheese.

Place the bowls under the broiler until the cheese is melted and bubbly.

Serve immediately and enjoy.