Heat the oil in a skillet over medium heat. Season chicken breasts on both sides with pepper and onion powder, and arrange in the skillet. Cook 2 minutes on each side, just until browned. Pour tomatoes and reserved liquid over the chicken. Pour in broth. Cover skillet, and continue cooking 15 minutes on each side, until chicken juices run clear.
While the chicken is cooking, place the frozen mixed vegetables and water in a pot. Scoop about 3/4 cup liquid from the skillet, and mix into the pot. Bring to a boil, and cook 5 minutes, or until vegetables are tender; drain. Arrange chicken breasts over the vegetables, and drizzle with liquid from the skillet to serve.