1. Place sliced leeks in a large bowl of cold water, and let soak for 5 to 10 minutes to rid them of dirt and sand. Lift them out of the water, and pat dry.

2. Heat oil in a medium nonstick skillet with an ovenproof handle over medium-low heat. Add leeks and cook until golden, about 7 minutes. Add tomatoes, 1/2 teaspoon salt, and 1/8 teaspoon pepper; stir. Raise heat to medium, and cook until tomatoes are tender, about 5 minutes. Remove from heat, and set aside to cool.

3. Heat broiler. Place 4 whole eggs in a large bowl, and beat with a fork. Add leek-and-tomato mixture, remaining 1 teaspoon salt and 1/8 teaspoon pepper, Parmesan, and basil; mix to combine.

4. Whisk egg whites until stiff but not dry. Gently fold whites into egg mixture.

5. Heat butter in a medium nonstick skillet with an ovenproof handle over medium heat. As soon as butter begins to foam, but not brown, pour in egg mixture. Reduce heat to low, cover pan, and cook until eggs start to set and thicken but the surface is still runny, 5 to 7 minutes.

6. Remove cover, and place skillet under the broiler. Broil until the top of the frittata is set, but not browned, about 3 to 5 minutes. Remove from broiler.

7. Run a spatula under frittata to loosen it from the pan, and transfer to a serving dish. Cut into 6 wedges, and serve immediately, or let stand for up to 1 hour, and serve at room temperature.