Preheat the oven to 350 degrees F. Coat a 9-inch deep-sided pie pan with nonstick cooking spray. Evenly sprinkle 2 tablespoons of the breadcrumbs into the pan.
Heat the oil in a large nonstick skillet over medium-low heat. Add the onion and 1/8 teaspoon salt and cover the skillet. Cook, stirring occasionally, until the onions soften without color, about 15 minutes. Stir in the chopped tomatoes and cook for 1 minute. Transfer to a medium bowl and set aside.
Meanwhile, add the eggs, egg whites, ricotta, milk, the remaining 1/3 cup breadcrumbs and 3/4 teaspoon salt to a blender until well combined and smooth. Stir in the sliced basil and the onion-tomato mixture.
Pour the egg mixture into the prepared pie pan. Sprinkle with the mozzarella. Arrange the sliced tomatoes in an overlapping style around the top.
Bake until the eggs are set and the cheese is lightly browned, about 35 minutes. Let stand for 10 minutes to complete the cooking process. Garnish with the fresh basil sprig. Slice into 4 wedges with a sharp knife and serve.