In a large Dutch oven, melt butter and olive oil over medium heat. Add sliced onions, thyme, bay leaves, salt, and pepper. Cook for about 10 minutes until softened.

Reduce heat to low and simmer onions for 40-45 minutes, stirring occasionally, until caramelized.

Stir in sherry wine and beef stock. Cook for an additional 6-8 minutes until the liquid reduces. Remove bay leaves and transfer to a bowl.

Preheat oven to 550°F. If using a pizza stone, place it in the oven to warm up.

Roll out pizza dough on a floured surface into a 10-inch circle. Transfer to a baking sheet or pizza peel.

Brush the dough with olive oil. Layer with Gruyere cheese, mozzarella, and then the caramelized onion mixture.

Bake for 10-12 minutes until the crust is golden brown and the cheese is bubbling.

Remove from oven, let cool for 5 minutes, slice, and serve warm.