Put the potatoes into a pan cover with cold salted water, Bring to the boil. Boil for 15 minutes until soft, drain well and mash, then add the butter and milk.
Put the carrots into a small saucepan, Cover with water and bring up to the boil. Boil for 10 minutes until soft, Drain well and puree using a stick blender.
Mix the mashed potatoes and carrot puree together.
Melt the 60g of butter in a saucepan. Add the onion and fry for 5-8 minutes until tender. Add the flour and stir over the heat for a few seconds. Whisk in the milk, whisking until smooth and thickened. Add the mustard, cheese and dill. Season well. Add the fish, mix well and spoon into an ovenproof dish.
Top with the potato and carrot topping.
Bake in the oven for 35 minutes until bubbling around the edges and then fish is cooked through.