Tzatziki Chicken

For sauce, mix the first 6 ingredients; refrigerate until serving., In a shallow bowl, whisk together flour, salt, pepper and baking powder. In another bowl, whisk together egg and milk. Pound chicken breasts with a meat mallet to 1/2-in. thickness. Dip in flour mixture to coat both sides; shake off excess. Dip in egg mixture, then again in flour mixture., In a large skillet, heat oil over medium heat. Cook chicken until golden brown and juices run clear, 5-7 minutes per side. Top with cheese. Serve with sauce and, if desired, lemon wedges.