Heat oven to 400°F for dark or nonstick pan (425°F for all other pans). Spray 15x10-inch pan with sides with cooking spray.
In 12-inch nonstick skillet, heat oil over medium-high heat. Stir in onion to coat with oil. Cook uncovered 10 minutes, stirring every 3 to 4 minutes. Reduce heat to medium-low. Cook 20 minutes longer, stirring well every 5 minutes; until onions are golden brown (onions will shrink during cooking). Set aside.
Unroll dough in pan; starting at center, press dough into 15x10-inch rectangle. Bake about 8 minutes or until light brown.
Spread pizza sauce over crust. Top with roast beef, onion, cheese and bell pepper.
Bake 8 to 10 minutes longer or until crust is golden brown and cheese is melted. To serve, cut into 6 rows by 3 rows