Spray counter lightly with cooking spray. Combine rolls and roll into a 12 x 16 inch rectangle. Cover with plastic and let rest while preparing filling.
Saute peppers, onion, mushrooms, garlic powder, salt and pepper in olive oil until tender. Set aside to let cool slightly. Remove plastic wrap from dough. Layer 10 slices of cheese on half of the dough lengthwise, leaving about an inch uncovered along sides and ends. Top cheese with roast beef. Layer vegetable filling on top of roast beef, mounding it to fit. Top with remaining cheese slices.
Brush the edge of the dough rectangle with water to help seal. Carefully fold the uncovered half of the dough over the top of the filling and press down firmly on edges to seal. When sealed on all three sides, carefully tuck sealed edges under stromboli. Gently lift onto sprayed baking sheet. Score slightly across top. Bake at 350°F 25-30 minutes.