Boil the potatoes ufor about 15-20 minutes until soft.
Mash potatoes adding a little bit of warmed skimmed milk.
Season well with salt & pepper and leave a side.
Bring 300 ml of skimmed milk to a simmer in another sauce pan.
Stir in the cornflour paste until it begins to thicken.
Reduce heat and season with salt and pepper.
Stir in the wholegrain mustard.
Add the cheese and stir until melted.
Remove from heat and leave a side.
In a pie dish, add the frozen haddock fillets.
Top with the diced carrots.
Pour 1/3 of the sauce mixture on top.
Top with the spinach.
Pour the rest of the sauce over.
Top with the mash and spread out evenly with a fork.
Brush with the beaten egg and bake at 180 degrees Celsius for 35 minutes until golden brown.