Ingredients 
1½kg vivaldi potatoes, peeled and cut into chunks 
450ml milk 
300ml double cream 
1 sprig of thyme
1 bay leaf 
½ tsp whole white peppercorns 
440g-500g white fish such as haddock, cod etc fillets, skinless * 
440g-500g salmon fillets, skinless * 
100g butter 
1 small onion, very finely chopped
50g flour
½ tsp white pepper 
5 tbsp finely chopped parsley 
150g raw king prawns 
1½ tsp salt 
25g Double Gloucester cheese, finely grated 

* When shopping at the supermarket fish comes in a variety of weights hence the difference here. Shorter, taller, fatter fillets work better than thin long pieces because of pan space but go with what you can get

Method
Put the potatoes into a saucepan, cover with water, add salt and bring to the boil over a medium heat. Reduce the heat to a simmer and cook until tender - about 15 to 20 minutes depending on the size of your potatoes. 

Meanwhile into a large pan put the milk, 250g of cream, thyme, bay leaf and white peppercorns. Gently stir and then add the white fish and salmon. Put the heat on medium and bring to the boil. As soon as the milk boils, turn off the heat and remove the fish to your oven dish.

Melt 50g of the butter in a medium saucepan over a medium heat. Add the onion and cook for 5-7 minutes until soft and translucent - you don't want to get any colour on the onions. Stir in the flour and cook for a minute, stirring continually. Remove from the heat and slowly incorporate the hot milk a little bit at a time until all of the milk is incorporated and the sauce free of flour lumps. Put the heat back on to medium and cook, stirring, until thick and smooth - around 5-10 minutes. Lower the heat and simmer for 5 minutes until thick and glossy, stir from time to time to stop a skin forming over the top. Add the parsley and season with salt and white pepper. You might not think there is enough sauce but there really will be.

Drain the potatoes and mash until completely smooth.  When you think they are smooth enough, mash a little more. Add the remaining butter and cream and stir through. Check the seasoning and adjust where necessary. If you think your potato is a little too stiff, add a touch of milk to slacken.

In your oven dish, drain out any liquid into your parsley sauce and make sure the fish is evenly distributed.  Add the prawns and top with the sauce.

Carefully spoon the potatoes over the fish, I do this with a spoon a little at a time starting around one edge as you don't want it to sink so take it slowly. Using a fork, rough up the top of the potato so you have lots of spiky bits - this is your texture. Top with the cheese.

 Put into the oven on a baking sheet as it has a habit of bubbling up and over and you won’t want to make a mess of your oven. Cook for roughly half an hour until the top is golden.

- An adapted recipe taken from The Ultimate by Angela Nielsen