01 - Preheat oven to 475°F. If using a pizza stone, place it in the oven to heat.
02 - In a large skillet over medium heat, add olive oil and butter. Once melted, add sliced onions, salt, and sugar. Cook, stirring often, for 20–25 minutes until golden brown and deeply caramelized. Stir in balsamic vinegar, if using, and cook for 1 more minute. Remove from heat and set aside.
03 - On a lightly floured surface, stretch or roll out pizza dough to a 12-inch round. Transfer to a parchment-lined baking sheet or a pizza peel dusted with cornmeal.
04 - Brush the dough lightly with olive oil. Evenly spread the caramelized onions over the dough, leaving a 1/2-inch border.
05 - Top with mozzarella, fontina (or Gruyère), and Parmesan cheese. Sprinkle with thyme leaves and black pepper.
06 - Bake for 12–15 minutes, until the crust is golden and the cheese is bubbling and browned in spots.
07 - Slice and serve hot.