Heat butter and oil in a large frying pan.
Add shallots, and saute until soft.
Lay fish on top of shallots.
Season, and add white wine.
Slowly bring to boil, and simmer for about 20 min.
turning fish once or twice.
When they are tender but not soft, transfer them to a hot serving dish, and keep warm.
Stir pan juices over a high heat until they have reduced.
Adjust seasoning, and add mustard and cream.
Gently heat for a moment (do not boil).
Pour over fish, and garnish with chives.