Melt butter in a heavy nonstick skillet over medium high heat. Saut? chicken about 5 minutes, turning once, or until tender. Remove chicken. Add onions to same pan and saut? 5 minutes. Reduce heat to low. Add half and half, parsley and mustard. Cook 2 minutes, stirring constantly, or until mixture is slightly thickened. Stir in lemon juice. Spoon 1 Tbs. sauce on each bottom muffin half. Top with chicken and remaining sauce and muffin halves.