1. Melt the butter in a large frying pan,
add the onion and cook for about 3 minutes,
stirring occasionally, until softened.
2. Stir in the cream and stock and bring to
the boil. Lower the heat, add the mace and
season lightly to taste. Gently add the fish
to the pan, loosely folded in half. Spoon the
cream over the fish. Cover and poach gently
for about 5 minutes, until the fish is tender
and just flakes.
3. Carefully transfer the fish to a warmed
plate, using a fish slice, cover and keep warm.
Boil the cooking liquor until slightly thickened.
4. Spoon the sauce over 4 warmed plates, discarding
the mace. Arrange the fish on top and garnish
with parsley sprigs and lemon twists.