Preheat oven to 180°C. Next, cook the potatoes and mash together with a splash of milk, parsley, salt and pepper (to taste) and a little butter.In a saucepan melt butter over a medium heat, then add diced onion and garlic. Blend in flour and cook for 3 minutes, stirring constantly.Slowly add in milk until the sauce thickens, then mix in mustard and season to taste.Stir in Smoked Fish Fillets and place mixture in an ovenproof dish. Top with potato mash then with grated cheese. Bake for 35 minutes.