Prepare the onions by peeling and thinly slicing them.
In a large saucepan, melt the butter with the olive oil.
Add the onions and cook over medium heat for 10 minutes until softened.
Sprinkle with sugar to help with caramelization, then bake for another 30 to 45 minutes until golden brown.
Add the minced garlic and sauté for a minute.
Stir in the sherry, deglaze the bottom of the pan, and cook until evaporated.
Pour in the beef stock, season with salt, add the bay leaf and thyme. Bring to a boil, then reduce the heat and simmer for 30 minutes.
Prepare the croutons by slicing the baguette, brush with olive oil and bake until golden brown.
Sprinkle the Gruyère cheese over the croutons and melt them under the broiler.
Remove the bay leaf and thyme sprigs from the soup. Season to taste.
Serve the soup in bowls, top with melted cheese croutons, and enjoy!
Advice:

Use yellow onions for better caramelization.
Do not increase the heat during caramelization to avoid burning the onions.
Prepare the croutons just before serving to keep them crispy.
The soup can be made ahead of time and reheated when ready to serve.
Add more Gruyere cheese if needed, as this gives a delicious flavor.