Bring wine and onion to simmer in heavy large skillet. Add fish, cover and simmer until fish is just cooked through, about 8 minutes. Transfer fish to plates. Tent with foil to keep warm. Add cream to skillet and bring to boil. Boil until reduced to sauce consistency, stirring occasionally, about 6 minutes. Mix in mustard. Spoon sauce over fish. Sprinkle with green onion tops.