1. Cut green pepper into bite-size pieces. Slice onion. Cut zucchini lengthwise into quarters; cut each quarter into 1/4" chunks. Set vegetables aside.
2. Cut each breast in half. With mallet, pound each piece to 1/4" thickness. On waxed paper combine flour, salt & pepper; coat chicken with mixture.
3. In 10" skillet, heat oil over medium-high heat; cook chicken until brown on both sides. Remove chicken. Using the drippings cook onion & pepper until crisp, stirring frequently.
4. Return chicken to skillet, add sauce, sugar & zucchini; over high heat, heat to boiling. Reduce heat, cover & simmer 15 minutes until chicken & vegetables are tender. Top with mozzarella, cover & simmer about 2 minutes.