This is a regular staple in our house, and I have been making this recipe pretty much unchanged for the past 4 years...I can guarantee my kids will always eat it!
500g fish fillets (I use salmon & cod but you can use whatever you like)
500ml whole milk
1 bay leaf
1 onion, peeled & diced
3 tbsp plain flour
1/2 tsp mustard powder
100g frozen peas
75g frozen shrimps
40g cheddar cheese, grated
1kg potatoes, peeled & chopped
Put the fish & the bay leaf in a shallow frying pan and pour over the milk. Bring to the boil then cook for 5 mins. Take out the fish fillets and put them into the ovenproof dish you are using, then flake them with a fork. Strain the milk through a sieve reserving the liquid in a jug, and discard the bay leaf.
Melt the butter in a pan and once melted, add the onion and saute for a few mins until soft. Stir in the flour to make a paste and cook for 1 minute. Add the strained milk a little at a time, continually stirring and letting the sauce thicken.
Stir in the mustard powder, peas, shrimps and cheese and cook through for 2 minutes. Pour the sauce over the flaked fish in your dish.
Boil the potatoes until soft, drain and mash with a generous knob of butter (and a little milk if they seem dry). Spoon the mash over the fish mixture, ensuring it reaches right to the edge of the dish.
Bake at 180 degrees for 25 mins.
I often have leftovers from this, and it keeps fine for 2 days in the fridge and also freezes well (assuming the fish is fresh not frozen originally).