Prepare the pasta according to the package directions. Drain and transfer to a 3-quart casserole dish.
Meanwhile, heat the oil in a large skillet over medium-low heat. Add the onion and garlic and cook for 5 minutes, or until softened. Add the zucchini or sweet red pepper, tomatoes, Italian seasoning, red-pepper flakes, salt and pepper. Cover and cook, stirring occasionally, for 10 minutes or until the vegetables are soft.
Combine the egg whites and the ricotta in a medium bowl, stir to mix well, and spoon over the pasta. Top with the vegetable mixture and sprinkle with the mozzarella and Parmesan.
Bake for 45 minutes, until the top is golden brown.