Pound each breast between sheets of plastic wrap to a uniform thickness of 1 cm. Spread with mustard. In a large skillet, melt butter over medium heat. Add chicken and cook 3-4 minutes on each side. Stir salsa and lime juice together and add to skillet. Simmer gently, uncovered, until chicken is cooked through and sauce has thickened, about 6-8 minutes. Sprinkle with cilantro. Serve with rice or noodles.