Preheat oven to 350 degrees. Place the chicken breasts in an ungreased 9x13 glass baking dish. Put the sliced onion and zucchini around the chicken. Sprinkle seasonings over the chicken and vegetables. Pour the tomato sauce and diced tomato over the top.
Bake for 30-35 minutes or until the chicken is cooked through. Periodically baste the chicken with the pan juices so the tomato sauce doesn't dry out on top.
Remove the chicken and vegetables to individual plates or a platter and toss either pasta or rice with the tomato sauce before serving.