Preheat the oven to 350°F. Brush both sides of the baguette slices with olive oil and place them in a single layer on a baking sheet. Bake for 10 minutes, flip, and bake for another 10 minutes until golden and crisp. Transfer to a wire rack to cool.

In a large Dutch oven or heavy-bottomed pot, heat the olive oil and butter over medium heat until melted. Add the onions and cook, stirring occasionally, until softened.

Add the salt, pepper, and sugar. Continue cooking until the onions are deep golden brown and jammy, about 45 to 50 minutes.

Stir in the thyme and sherry or wine. Cook until the liquid has mostly evaporated, then add the beef stock and bay leaves.

Bring to a boil, then reduce the heat and simmer for 10 minutes. Taste and adjust the seasoning as needed.

Ladle the soup into oven-safe bowls. Top each bowl with a slice of toasted baguette and a generous amount of cheese.

Broil on low until the cheese is melted, bubbling, and browned in spots. Serve immediately.

Make-ahead note: The soup base can be made ahead and stored in the refrigerator for up to 2 days or frozen for up to 2 months. Reheat gently, then add the bread and cheese just before broiling.