In a shallow bowl, mix flour, salt and pepper. Pound chicken breasts with a meat mallet to 1/2-in. thickness. Dip chicken in flour mixture to coat both sides; shake off excess., In a large skillet, heat butter over medium heat. Brown chicken on both sides. Add wine; bring to a boil. Reduce heat; simmer, uncovered, until chicken is no longer pink, 12-15 minutes. Drizzle with lemon juice. If desired, sprinkle with parsley.