Preheat oven to 350°.
Melt butter in a Dutch oven over medium heat. Add leek; sauté 4 minutes. Add spinach; sauté 2 minutes or until spinach wilts. Place mixture in a colander, pressing until barely moist.
Combine milk, basil, salt, pepper, eggs, and egg whites; stir well with a whisk. Add leek mixture and potato. Pour into a 10-inch round ceramic baking dish or pie plate coated with cooking spray. Sprinkle with breadcrumbs and top with cheese. Bake at 350° for 25 minutes or until center is set.
Broil frittata 4 minutes or until golden brown. Cut into wedges.