01 - Preheat your oven to 475°F and place a pizza stone or heavy baking sheet inside to heat thoroughly while you prepare the toppings.
02 - In a large skillet over medium heat, melt butter with olive oil. Add onions, salt, and sugar. Cook, stirring occasionally, for 20-25 minutes until onions are deeply golden and caramelized. Stir in black pepper and balsamic vinegar. Remove from heat and set aside.
03 - On a lightly floured surface, stretch or roll the pizza dough into a 12-inch circle. Transfer to a piece of parchment paper for easy transfer to the oven.
04 - Brush the dough lightly with olive oil. Evenly spread the caramelized onions over the base. Sprinkle mozzarella, fontina (or Gruyère), and Parmesan cheeses over the onions. Top with thyme if using.
05 - Carefully slide the pizza (with parchment) onto the preheated stone or baking sheet. Bake for 12-15 minutes, or until the crust is golden and the cheese is bubbling and lightly browned.
06 - Remove from oven and let cool for 2 minutes. Garnish with fresh arugula or basil if desired. Slice and serve immediately.