Preheat oven to 425°F.
Preheat a large skillet over medium-high heat with the butter and 2 turns of the pan of EVOO, about 2 tablespoons. Add the sliced onions, season with salt and pepper, and cook; stirring frequently for 20-25 minutes or until very brown and caramelized. Remove from skillet and let cool slightly.
Drizzle a little EVOO on to the pizza dough and pull and stretch until almost the same size as the baking sheet pan. Drizzle the baking sheet pan with EVOO, then set crust in place. Poke the pizza dough in several areas with the tines of a fork. Season the dough with salt and pepper, and spread the caramelized onions on top. Top the onions with the prosciutto, then the fontina and gorgonzola cheese.