In a medium (10-inch) nonstick broilerproof skillet, heat 1 tablespoon oil over medium heat. Add onion, potato, and rosemary; season with salt and pepper, and toss to combine.
Cover skillet, and cook 10 minutes; uncover. Cook, tossing mixture occasionally, until onion and potato are tender, about 5 minutes.
Meanwhile, in a medium bowl, whisk together eggs, egg whites, parsley leaves, 3/4 teaspoon salt, and 1/4 teaspoon pepper.
Heat broiler with rack set 4 inches from heat. Add remaining tablespoon oil to vegetables in skillet. Pour egg mixture into skillet.
Cook on stove, over low heat, lifting mixture a few times around the edges with a spatula to let egg flow underneath. Continue cooking until frittata is almost set in center, about 10 minutes.
Place skillet under broiler; broil until frittata is set and top is lightly golden, about 3 minutes. Run a clean spatula around edges to loosen, then slide frittata out onto a serving plate, and cut into wedges.
Extra yolks: To store (for custards or egg washes), place unbroken yolks in water in a covered container and refrigerate, up to 2 days. If you have no use for them, you can buy unprocessed pure egg whites in the dairy section at the market.