In a large pot, melt the butter and olive oil over medium heat.

Add the sliced onions and sugar, and cook, stirring frequently, until the onions are caramelized and golden brown, about 25-30 minutes.

Stir in the minced garlic and cook for an additional minute.

Sprinkle the flour over the onions and stir to combine, cooking for another 2 minutes.

Gradually add the beef broth and water, stirring constantly to prevent lumps.

Add the thyme, bay leaf, salt, and pepper. Bring the soup to a simmer and let it cook for about 30 minutes, stirring occasionally.

While the soup is simmering, preheat the oven to 350°F (175°C).

Place the sliced French bread on a baking sheet and toast in the oven until golden brown, about 10 minutes.

Remove the bay leaf from the soup, then ladle the soup into oven-safe bowls.

Top each bowl with a slice of toasted French bread and a generous amount of grated Gruyère cheese.

Place the bowls on a baking sheet and broil in the oven until the cheese is bubbly and golden, about 3-5 minutes.

Garnish with fresh parsley before serving.