In an oven-safe skillet, melt the butter. Sear fish and remove from pan.
Add leeks, onion, and celery. Sweat five minutes, deglaze with wine. Add water and simmer.
Add fish back in, cover and bake for 15 minutes at 350 degrees.
Remove fish, strain liquid into saucepan and reduce by half.
Add mustard and tarragon and whisk. Add cream and reduce by half.
Serve with fish and warm saffron couscous.