Preheat oven to 400° and line a large rimmed baking sheet with parchment paper.
Heat 2 tablespoons of butter in large skillet over medium heat. Add sliced onions and cook until soft and caramelized, about 10 minutes. Remove from heat and set aside. Remove pizza dough from package and place on prepared baking sheet. Roll out into any desired freeform shape. Heat remaining butter and garlic in small microwave safe bowl for approximately 30-45 seconds. Remove and add fresh parsley. Brush entire raw dough with butter and garlic mixture. Place quartered tomatoes in an even layer on dough, followed by caramelized onions. Sprinkle with mozzarella and fontina cheeses. Top with slices of pancetta.
Bake for 15-20 minutes, or until cheese is bubbly and crust is golden. Remove from oven and transfer to large wooden serving board. Garnish with fresh basil. Cut into bite-size squares.