Cut the fish fillets into 2cm cubes, wash and de-beard the mussels (pull the piece of hair-like material towards the fat end of the mussel and remove it). Discard any mussels that are cracked or remain open after tapping sharply.
Warm the milk gently in a saucepan. In another saucepan, melt the butter and add the diced onion. Add the vegetable oil to stop the butter from burning. Once the onions are soft, add the flour and stir until mixed through. Gradually add the warm milk, one cup at a time, stirring thoroughly until smooth. When all of the milk is added, add a knob of butter and stir in.
To finish, add the fish pieces, cooked diced potatoes (if using) and mussels, and gently cook until the mussels have opened, which should be approximately 4 to 6 minutes. Check seasoning at the end as the smoked haddock may have seasoned the chowder enough or add salt and pepper to taste. Add chopped dill and parsley and sprinkle with paprika to serve.
Thanks to our friends at Seafood Scotland for this recipe!