Add the butter to a shallow casserole dish with onions, mustard, lemon juice and a pinch of salt and pepper.
Stir over medium heat until the onions are softened, but not browned.
Add the prawns, smoked haddock, and chicken stock. Bring to a simmer and pop the lid on the dish.
Once the liquid has reduced by about half, stir the fish, and season with salt and pepper to taste.
Stir the crème fraîche evenly on the liquid mixture and scatter the chopped chives. Bring to a gentle simmer.
Finish the dish with a final squeeze of lemon juice.
Serving Suggestions: Pour into the serving dish and sprinkle with chopped chives. Serve with mashed potato or basmati rice.