For sauce, melt butter over medium heat in a medium saucepan. Add onion and garlic, cook until tender. Add tomatoes, sugar, salt, and pepper and stir. Bring to boiling and reduce heat. Simmer about 10 minutes or until desired consistency, stirring occasionally. Stir in basil. 

Meanwhile, using the flat side of a meat mallet, flatten chicken between two pieces of plastic wrap until about ¼ inch thick. 

In a shallow bowl, stir together bread crumbs, 3 tablespoons of the Parmesan cheese and oregano. In a second bowl, stir together egg and milk. Dip chicken into egg mixture; coat evenly with crumb mixture. 

In a 12-inch skillet heat oil over medium heat. Cook chicken for 4 to 6 minutes or until golden, turning once. 

Spoon sauce over chicken. Top with mozzarella and the remaining 1 tablespoon of Parmesan. Let stand 2 minutes. If desired, serve with hot cooked pasta.