• First up, you’ll need a good-sized, ovenproof gratin dish. I used a 14-inch, cast-iron oval one which is about two inches or so deep. A similarly sized ceramic gratin dish is also dandy.
• Heat your oven to 400F / 200C.

Make the mashed potatoes

• Put the potatoes and garlic into a medium-sized pan and just cover them with water. Add a heaped teaspoon salt and bring the pan to a boil. As soon as it does, drop the heat to low, cover the pan and let the potatoes cook right through – about ten minutes. Then drain the potatoes and garlic in a big colander, return them both to their pan and set them aside to cool. Do not discard the garlic.
• Peel the potatoes as soon as they are cool enough to handle. Add a level tablespoon butter, a heaped teaspoon salt, one of black pepper, and all the Parmesan. Now mash the lot until the potatoes are just barely smooth. Cover the pan and set it aside.

Cook the fish

• Set a big skillet (12-inch is grand) on a medium heat and add the milk, bay leaves, and the two types of fish fillets. As soon as the milk just begins to boil, drop the heat to low. The skillet will be pretty crowded by all the fish, but that’s fine because it means they’ll sit deeper in the milk.
• You want the fish to poach in the milk at a very gentle simmer – and I mean very gentle - for two minutes on each side. So that’s a total of four minutes at that low simmer. After those four minutes, the poaching is finished and you can turn off the heat.
• Using a big slotted spoon, remove all the fillets and set them aside in your gratin dish. Leave the milk and the bay leaves in the skillet – that’s important because it’s all going into the sauce.

Make the sauce

• Melt the butter in a medium-sized saucepan on a low heat. Now add the flour and mix it very thoroughly with the butter on that low heat. Then stir in all the milk from the skillet together with the bay leaves. Keep stirring so that the milk becomes completely combined with the butter and flour mix. Add the heavy cream and stir, stir, stir until you have a completely smooth mixture. You might want to use a small whisk to speed things up a bit, but I just like a wooden spoon.
• Now, still on that low heat, stir into the creamy sauce the chopped serrano chili, black pepper, capers, and all the tarragon, parsley, and dill. Now add the prawns and, very gently, stir them into the sauce. Continue to cook on low for another three minutes, with the occasional gentle stir so as not to break the prawns. Done. Check for saltiness and adjust accordingly to your taste. Sauce finished.

Bringing it all together

• Use your fingers to break the fish fillets apart in the gratin dish. You want the pieces to be quite chunky – a sort of big bite size but certainly not smaller. Space the fillets apart so you have a fairly even spread of white fish and smoked fish.
• Now pour the sauce – with the prawns and bay leaves – over the fish. Stir gently so as not to break the fish apart and to get the prawns evenly spaced across the gratin dish. Time for the topping.
• Using a dessert spoon, start sliding spoonfuls of the mash onto the sauced seafood. Take some care doing this. You want – as much as possible – for the mash to sit on top, rather than being pressed down into the seafood. Once the dish is evenly covered, use a fork to create furrows right across the top of the mash. Sprinkle on a few pinches of paprika, and put the dish on a high shelf into your pre-heated oven.
• Let it bake for 35 minutes. The sauce may well bubble out a little around the edges of the dish – that’s fine. And the topping will have taken on some golden crunch. And that really is grand.
• Serve at once with those lovely green peas.