Place potato and 3 tablespoons water in a small microwave-safe bowl. Cover with plastic wrap, and vent. Microwave at high 4 minutes or until tender, stirring once. Set aside.
Heat a 10-inch nonstick skillet over medium-high heat. Coat pan with cooking spray. Add onion. Cover and cook 10 minutes or until lightly browned, stirring occasionally. Uncover and cook 10 minutes or until golden brown, stirring frequently. While onion cooks, add 1/3 cup water, 1 tablespoon at a time, to prevent onion from sticking to pan. Stir in potato, sage, salt, and pepper. Spoon into a medium bowl; cool slightly.
Combine 3 tablespoons cheese, milk, egg whites, and eggs in a small bowl; stir with a whisk. Add egg mixture to potato mixture; stir well.
Wipe pan with a paper towel; recoat pan with cooking spray. Pour potato mixture into pan. Cook over medium heat 7 minutes or until bottom of frittata is browned and top is almost set.
Wrap handle of pan with foil. Sprinkle remaining 1 tablespoon cheese over frittata. Broil 5 minutes or until cheese melts and top is set. Cut into wedges.