Slice the potatoes into 1/4-inch thick pieces.

Chop the red onions and sweet pepper.

In a covered 6- to 7-inch nonstick skillet, cook potatoes and onion in a small amount of boiling water for 7 minutes.

Add sweet pepper to the skillet.

Cook, covered, for 3 to 5 minutes more or until vegetables are tender.

Drain the vegetables in a colander.

In a small bowl, whisk together egg, rosemary, salt, and pepper.

Set the egg mixture aside.

Wipe out the skillet and lightly coat with cooking spray.

Return the vegetables to the skillet.

Pour the egg mixture over the vegetables.

Cook over medium heat, without stirring, for about 1 minute or until the egg mixture begins to set.

Run a spatula around the edge, lifting the mixture so the uncooked portion flows underneath.

Continue cooking and lifting edges until the egg is almost set but still glossy and moist.

Remove the skillet from heat.

Sprinkle with cheese.

Let stand, covered, for 3 to 4 minutes or until the top is set and cheese is melted.

Cut frittata into wedges to serve.

Garnish with fresh rosemary, if desired.