1. Place each chicken breast half between 2 sheets of heavy-duty plastic wrap, and flatten to 1/4-inch thickness using a meat mallet or rolling pin.
2. Stir together egg and 1/4 cup water.
3. Combine breadcrumbs and Parmesan cheese. Dip chicken in egg mixture; dredge in breadcrumb mixture.
4. Melt 1 tablespoon butter in a large skillet over medium heat; add chicken breast halves, and brown on each side.
5. Arrange chicken in a lightly greased 11- x 7-inch baking dish or individual gratin dishes.
6. Melt remaining 1 tablespoon butter in skillet over medium heat. Add onion and garlic; sauté 2 minutes or until tender. Add tomatoes; cook 1 minute. Pour mixture over chicken. Sprinkle with mozzarella cheese.
7. Bake at 350° for 20 minutes or until thoroughly heated.