Cut the onion into very thin slices. Put the vegetable oil and onions into a large skillet and place it over medium-low heat. Cook, stirring frequently, until the onions become golden brown and caramelized, about 30 minutes.
Place a pizza stone onto the floor of the oven and preheat the oven to 450 degrees F.
Remove the bones from the duck legs and shred the meat; set aside. Place the dough on a floured surface. Flatten it slightly, sprinkle with more flour, and roll it into a 10-inch disk. Sprinkle a wooden pizza paddle with flour and transfer the dough to the paddle. Shake the board a little to make sure the dough doesn't stick. Spread the caramelized onions to within 1/2-inch of the edge of the dough and top evenly with the cheeses, pine nuts and rosemary. Drizzle with extra-virgin olive oil and transfer to the pizza stone. Bake until the crust is brown around the edges, about 10 to 15 minutes.
Recipe courtesy of Dan Smith and Steve McDonagh