1. Heat 1 tablespoon oil in large nonstick skillet. Add onions, mushrooms, sweet pepper, salt, and pepper. Cook 7 to 8 minutes, until vegetables are tender. Transfer to bowl.
2. Whisk eggs, water, and rosemary in a bowl. Add remaining 1 tablespoon oil in a bowl. Add remaining 1 tablespoon oil to skillet on low heat. Add half the egg mixture, then vegetables in an even layer; top with cheese and remaining egg mixture. Cover and cook 9 to 10 minutes, until egg is set at edges and slightly loose in center. Place large plate over top of skillet; holding bottom of plate, invert skillet and unmold frittata onto plate. Slide frittata back into skillet cooked side up; cook 1 to 2 minutes more. Makes 4 servings.