Flatten chicken to 1/4-in. thickness. Combine spinach and 1/2 cup feta cheese; spoon down the center of each chicken breast half. Fold chicken over filling and secure with toothpicks. Combine the flour, oregano, salt and pepper; rub over chicken. , In a large skillet, brown chicken in oil on all sides. Discard toothpicks. Place chicken in a greased shallow 3-qt. microwave-safe dish. Top with tomatoes and remaining feta cheese. , Cover and microwave on high for 18-20 minutes or until chicken juices run clear and cheese is melted. Serve over pasta if desired.