Whisk together first 6 ingredients until well combined; stir in capers and parsley. Use immediately, or cover and chill until ready to use. Store in an airtight container in refrigerator up to 3 days. If chilled, whisk before using.
Italian Pan-Cooked Chicken Breasts: Pour Italian-Mustard Marinade into a shallow dish or large zip-top plastic freezer bag; add 4 (7- to 8-oz.) skinned and boned chicken breasts. Cover or seal, and chill at least 8 hours or up to 24 hours, turning occasionally. Remove chicken from marinade, discarding marinade. Pat chicken dry, and sprinkle with 1 tsp. salt and 1/2 tsp. pepper. Cook chicken in 1 to 2 Tbsp. hot olive oil in large skillet over medium heat 6 to 8 minutes on each side or until done. Makes 4 servings; Prep: 10 min., Chill: 8 hr., Cook: 16 min.