- Preheat oven to 200°C.
- Melt butter over medium heat in a saucepan, remove and stir in the flour to form a roux. Return to the heat and cook for 30 seconds, stirring continuously.
- Gradually add the milk a bit at a time, whisking as you go, to stop lumps forming.
- When all milk is incorporated and sauce has a creamy consistency add mustard and nutmeg, three quarters of the grated cheese and a squeeze of lemon juice, season with salt and pepper to taste.
- Prepare spinach; remove tough stems, wash well, drain, tear up roughly, place in a pan of lightly salted boiling water and cook for 1 to 2 minutes. Drain, then squeeze out excess liquid using a sieve and the back of a spoon.
- Spread cooked spinach evenly across the bottom of an oven proof baking or casserole dish, place haddock fillets on top of it, then pour cheese sauce over fish and sprinkle remaining grated cheese over the top.
- Bake 15 to 20 minutes until golden on top. To complete the browning of the cheesy top if it hasn’t browned to your satisfaction already, turn on the grill and grill for about a minute.
- You can also top this dish with mashed potatoes to turn this bake into a delicious fish pie.