Place the sliced yellow onions, ground black pepper, butter, and olive oil in your crockpot.

Set the crockpot to low and cook the onions for 12 hours overnight. Stir once or twice if possible to ensure even caramelization. The onions will soften, become golden, and develop a sweet, rich flavor.

After the 12 hours, add the beef broth to the crockpot. Stir well to combine.

Continue cooking on low for an additional 6-8 hours to allow the flavors to meld and deepen.

Preheat your oven to 350°F (175°C).

Spoon the soup into individual broiler-safe bowls — ceramic or stoneware usually work best.

Top each bowl of soup with a slice or two of French baguette and generously sprinkle with grated Gruyère cheese.

Place the soup bowls on a baking sheet and transfer them to the oven.

Broil for 5-7 minutes or until the cheese is melted, bubbly, and golden brown. Keep a close eye on the soup to ensure the cheese doesn't burn.

Carefully remove the bowls from the oven, allow them to cool slightly, and serve immediately.