Begin by caramelizing the onions. In a large skillet, heat the olive oil and butter over medium heat. Add the sliced onions, sprinkle with salt and sugar, and cook, frequently stirring, until the onions are golden brown and sweet, about 20 minutes. Set aside.
Preheat your oven to 475°F (245°C). If you have a pizza stone, place it in the oven to heat as well.
On a floured surface, stretch or roll out your pizza dough to a 12-inch (30 cm) circle or your desired thickness.
Transfer the dough to a pizza peel dusted with cornmeal or to a baking sheet if you don’t have a pizza stone.
Evenly distribute the caramelized onions over the pizza dough, leaving a small border around the edges.
Sprinkle the crumbled Gorgonzola and shredded mozzarella cheeses over the onions. Season with freshly ground black pepper.
Slide the pizza onto the preheated pizza stone or place the baking sheet in the oven. Bake for 10-12 minutes or until the crust is golden and the cheese is bubbly.
While the pizza is baking, prepare the balsamic glaze. In a small saucepan over medium heat, combine the balsamic vinegar and brown sugar. Stir until the sugar is dissolved, and simmer until the mixture is reduced by half and has a syrupy consistency, about 10 minutes.
Once the pizza is done, remove it from the oven and let it cool for a few minutes.
Drizzle the balsamic glaze over the pizza just before serving.