In a covered 6- to 7-inch nonstick skillet with flared sides, cook potatoes and onion in a small amount of boiling water for 7 minutes. Add sweet pepper. Cook, covered, for 3 to 5 minutes more or until vegetables are tender. Drain in a colander.
Meanwhile, in a small bowl, whisk together egg, 1/2 teaspoon rosemary, salt, and pepper. Set aside.
Wipe out skillet; lightly coat with cooking spray. Return vegetables to skillet. Pour egg mixture over vegetables. Cook over medium heat, without stirring, about 1 minute or until egg mixture begins to set. Run a spatula around the edge, lifting mixture so uncooked portion flows underneath. Continue cooking and lifting edges until egg is almost set but still glossy and moist.
Remove skillet from heat. Sprinkle with cheese. Let stand, covered, for 3 to 4 minutes or until top is set and cheese is melted.
To serve, cut frittata into wedges. If desired, top each servings with fresh rosemary.