1. Place chicken between two sheets of heavy-duty plastic wrap; flatten to 1/4-inch thickness, using a meat mallet or rolling pin.
2. Dip chicken in egg, and dredge in breadcrumbs.
3. Cook chicken in butter in a large skillet over medium-high heat until browned on both sides.
4. Spoon spaghetti sauce over chicken; bring to a boil.
5. Cover, reduce heat, and simmer 10 minutes.
6. Sprinkle with cheeses and parsley; cover and simmer 5 additional minutes or until cheeses melt.