Preheat oven to 400°F. Warm oil in a skillet over medium high heat. Add onion and sauté until translucent, 5 minutes. Add bell pepper, mushrooms, garlic and Italian seasoning; sauté until mushrooms are lightly browned, 8 minutes. Stir in soup; cook until slightly thickened, 2 minutes.
Spoon vegetable mixture into a shallow 3-quart, 13-by-9-inch baking dish. Top with pepperoni, reserving slices for garnish, and sprinkle with 1 1/2 cups cheese.
Mist edges of dish with nonstick cooking spray. Roll out or stretch pizza dough to fit top of pan, trimming excess; pinch edges against pan to seal. Bake for 5 minutes. Remove from oven and cut slits in dough to release steam. Sprinkle with remaining cheese and top with reserved pepperoni. Return to oven and continue baking until top is risen and nicely browned, about 20 minutes longer. Let stand 5 minutes before serving.