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.