Heat 4 tsp (20 mL) olive oil over medium-low heat in a large skillet. Add 2 cups (500 mL) sliced yellow onions and cook until soft and golden brown, 20 to 30 min.
Meanwhile, poke 10 potatoes a few times with the tip of a knife and microwave on HIGH for 1 1/2 to 2 min. or until softened. Cool potatoes, then cut into 1/4-in. (5 mm) slices.
Place 2 pizza crusts on a lightly oiled grill heated to medium. Cook for 2 min. or until bubbly and rigid. Using tongs, turn crusts over. Divide cheese, potatoes and onions between pizza crusts. Close lid and cook until cheese is bubbly and crusts are golden brown, 2 to 3 min.
Remove from grill and season with 2 tbsp (30 mL) finely chopped fresh rosemary, 1/2 tsp (2 mL) each salt and pepper. Drizzle with 2 tsp (10 mL) olive oil. Slice each pizza into 6 wedges and serve.