In a large nonstick skillet, saute the peppers, mushrooms and onion in oil until tender. Stir in garlic powder and pepper; set aside.
On a baking sheet coated with cooking spray, roll dough into a 15x10-in. rectangle. Layer half of the sliced cheese, all of roast beef and vegetable mixture lengthwise over half of dough to within 1/2 in. of edges. Top with remaining sliced cheese. Fold dough over filling; pinch seams to seal and tuck ends under.
Combine egg white and water; brush over dough. Cut slits in top. Bake at 350° for 20-25 minutes or until golden brown. Sprinkle with mozzarella cheese. Bake 5 minutes longer or until cheese is melted. Let stand for 10 minutes before cutting.