COAT fish with mustard. Heat oil in medium skillet on medium-high heat. Add fish; cook 3 min. on each side or until browned on both sides. Remove fish from skillet; cover to keep warm.
ADD onions and garlic to skillet; cook 3 min., stirring occasionally. Add all remaining ingredients except 1 Tbsp. parsley. Cook 5 min.; stir. Top with fish; cover. Cook 4 to 5 min. or until fish flakes easily with fork.
SPRINKLE with remaining parsley.