Heat a large skillet over medium heat. Spray with cooking spray. Sprinkle chicken with 1/4 tsp each salt & pepper. Add chicken to pan, cook 5 minutes. Turn and cook 2 minutes. Remove chicken from pan.
Add onion and garlic to the pan and saute 2 minutes. Add remaining 1/4 tsp salt & pepper, diced tomato, marinara sauce and oregano to the pan. Return chicken to the pan. Sprinkle fresh mozzarella pieces over the top of the chicken.
Place in the oven for 6-8 minutes (and make certain chicken is done as cooking time vary with thickness!)
Let cool a bit and serve.