Fish and chips with no-waste shell curry sauce

Method

1. To make the sauce, put the shells in a large heavy bowl and use the end of a rolling pin to bash into smaller pieces. Heat 1 tbsp of the oil in a large saucepan over a medium-high heat, then add the shells, shallots, carrot, fennel, garlic and a large pinch of salt. Cook for 15-20 minutes, stirring regularly, until golden and starting to caramelise.
2. Stir in the curry powder and cook for 1 minute. Add the wine and bubble until it’s almost all reduced, then add 1 litre water, the bay leaf and peppercorns. Bring to the boil, then reduce to a simmer and bubble gently for 1 hour, stirring occasionally.
3. When the sauce has had an hour, pass it through a sieve into a medium saucepan, then return to the heat and boil to reduce by two-thirds (about 20 minutes).
4. Meanwhile, prepare the chips. Heat the oven to 190°C fan/gas 6½ with a tray inside to heat up. Cut the potatoes into 1cm thick chips and rinse in cold water. Tip into a saucepan, cover with salted water, bring to the boil, then simmer for 5 minutes. Drain and leave to steam dry. Add half the oil to the tray in the oven once it’s hot, then put the tray back in the oven for 3 minutes. Season the chips with salt, then tip carefully into the hot oil and turn to coat. Roast for 30-35 minutes, turning halfway, until crisp and golden. Season with salt flakes as soon as the chips come out of the oven.
5. Put the fish on a plate and dry a little with kitchen paper. Season with salt and leave for a few minutes. Dust with the flour and paprika, then season with pepper.
6. Once the sauce has reduced, turn the heat to low and stir in the cream. Add the butter, a few cubes at a time, stirring all the time, until melted and emulsified into a glossy sauce. Season to taste, then keep warm over a low heat.
7. Bring a small pan of salted water to the boil and cook the peas for 3 minutes. Drain and return to the pan. Add the butter and half the tarragon, then mash with a potato masher. Season and cover to keep warm.
8. Heat a nonstick frying pan over a medium heat and add the remaining 1 tbsp oil. Once hot, add the fish fillets. Cook for 2-4 minutes (depending on  thickness) until the edges are starting to turn opaque. Flip and cook for another 2-4 minutes until the flesh is opaque all the way through.
9. Serve the fish, chips and mushy peas with the curry sauce for pouring over. Add a wedge of lemon for squeezing.