Coat a large skillet with nonstick cooking spray; heat over medium-heat. Add the chicken, onion, and garlic, and sauté for 5 to 6 minutes, stirring until the chicken is browned on all sides.
Stir in the beans, tomato, salsa, salt, and pepper. Cook for 1 to 2 minutes, or until heated through and no pink remains in the chicken.
Spoon over warm rice, if desired, and serve topped with a dollop of sour cream.