Steps
1. In a large pot, melt the butter over medium-high heat.
Add the onion and sugar, and cook for about 10 minutes, stirring occasionally until the onions are golden and caramelized.
2. Stir in the flour until combined, then slowly add the beef broth.
Season with salt and pepper and bring to a boil. Reduce heat and simmer for 15 minutes.
3. Preheat the oven to 425°F.
4. Place two oven-proof bowls on a baking sheet. Ladle the soup into each bowl and top with a slice of Gruyere cheese and a slice of French bread.
5. Bake for 10 minutes, or until the cheese is melted and bubbly. Serve hot.