All in one fish pie

This is a real comforting winter supper.  It might look like a bit of work, but if you follow the order and hints below, it will be ready in under an hour.  You do not need anything with it, maybe just a few slices of fresh tomatoes, if you really have to add something.  (Try to use fresh fish.  If you only have frozen fish, make sure it is completely defrosted and pat it dry with paper towel to remove any excess water.)

4 potatoes

1 sweet potato

1 onion, finely sliced

2 carrots, diced

150g white mushrooms, diced

1 tablespoon butter

1 tablespoon olive oil

1 heaped tablespoon flour

300ml full cream milk or half milk and half cream

1 teaspoon prepared mustard

juice of one lemon

1 cup grated cheddar cheese

salt and black pepper

nutmeg

3/4 cup frozen peas (deforsted by pouring over some boiling water) 

500g smoked haddock, skin removed and sliced into 2 cm strips

Preheat the oven to 220 degrees.  Firstly, prepare the fish, then peel and thinly slice the potatoes and sweet potato (by slicing them thinly the cooking time is reduced).  While doing that, bring a saucepan of water to the boil.  Put the potatoes and sweet potatoes with a pinch of salt  into the boiling water and cook for 8 - 10 minutes, until soft. 

In the meantime prepare the onion, carrots and mushrooms.  Heat the butter and oil and saute the onion and carrot for about 5 minutes, then add the mushrooms and saute for another minute.  Sprinkle with flour and stir through.  Slowly add the milk/cream and bring to the boil and boil for a minute to thicken.  Remove from the heat and add the mustard, lemon juice and cheese.  Season with salt and pepper.

When the potatoes are cooked, drain and mash and add a knob of butter and season with salt, pepper and nutmeg.

Mix the fish, sauce and peas and spoon into an ovenproof dish.  Top with mash and bake for 30 minutes until golden.