Preheat oven to 375 degrees F. Place each chicken breast between plastic wrap; lightly pound with flat side of meat mallet to about 1/4 inch thickness.
In bowl stir together egg white, spinach, cottage cheese, and half the mozzarella; spoon on chicken, leaving 1/2-inch border. Roll up from narrow side. Place, seam sides down, in 2-quart rectangular baking dish. Combine spaghetti sauce and tomato paste; spoon over chicken. Bake, covered, for 25 minutes. Uncover; sprinkle with remaining mozzarella. Bake, uncovered, about 25 minutes more or until chicken is no longer pink (170 degrees F) and cheese is light brown. Let stand 10 minutes. If desired, serve with spaghetti. Serves 4.