Heat the oven to 200°C/180°C fan. Peel and cut the potatoes into evenly sized chunks.

Put the potatoes in a saucepan and cover with water, bring to the boil and cook for 15-20 minutes until very tender. Drain, then return to the pan over a medium heat for 30 seconds to dry.

Grate the cheese and finely chop the parsley.

Mash the potatoes with half the butter, one third of the milk and two thirds of the cheese. Season with salt and pepper.

In a shallow casserole over medium heat, melt the remaining half of the butter. Add the flour to the pan, cook for a minute or two then gradually stir in the remaining milk and cream to form a thick, shiny, sauce.

Stir in the mustard and season to taste.

Turn off the heat, then stir in the spinach and allow to wilt.

Mix the prawns, fish, capers, and parsley into the sauce and stir well.

Top with the mashed potato, then scatter over the remaining cheese.

Bake for 30-35 minutes until golden and crisp and the filling is piping hot.