Preheat the oven to 200'C / Fan 180'C / Gas Mark 4. Peel the potatoes and roughly chop into chunks. Add to a pan of cold, salted water, bring to the boil and cook until tender when poked with a knife (around 20-25 mins depending on the size of the chunks).

While the potatoes cook, make the sauce. Melt the butter in a large, non-stick saucepan over a low heat. Add the gluten free flour and stir until it forms a thick paste. Cook, stirring continuously, for around 30 seconds - 1 minute.

Pour the white wine (if using) into a jug with the milk, then start to add the milk/wine mix to the pan a little at a time. Add around 100ml of milk and whisk until it forms a paste with no lumps. Continue to do this until all the milk/wine mixture is gone and you have a thick, lump-free sauce. Make sure you constantly whisk while doing this to avoid any lumps or the sauce sticking.

Add the tomato ketchup, dijon mustard and parsley to the sauce, season generously with salt and pepper and then stir to mix. Once mixed, add the fish, prawns and frozen peas to the mixture. Stir well and then remove from the heat.

When the potatoes are done, drain them and then add a knob of butter and 75g of the cheese to the pan. Mash using a potato masher or stick blender until they are smooth.

Pour the fish sauce mixture into a large ovenproof dish and spread out evenly. Next, spoon the mashed potato on top and use a fork to spread it evenly over the fish, so there are no gaps. Smooth down the top and sprinkle over the remaining grated cheese.

Place the pie in the oven and bake for 30-35 minutes, until the mashed potato is golden and crisp on top. Remove from the oven and serve hot with some steamed vegetables.