First, make the stocks. For the fish stock, preheat an oven to 200°C/gas mark 6. Line a roasting tray with silicone paper and lay the fish bones and/or cod heads on top. Roast for 30 minutes, then turn the bones over and roast for another 10 minutes

Transfer the roasted bones and/or heads to a stockpot and pour over enough cold water to cover. Bring to a simmer over a medium heat and skim off any impurities from the surface. Allow to simmer for 30 minutes. Remove from the heat and strain the stock through a sieve into another pan. Bring back to a simmer and reduce by half, then remove from the heat and allow to cool

For the shellfish stock, preheat the oven to 200°C/gas mark 6. Place the shore crabs on a roasting tray and cook for 45 minutes

Meanwhile, heat a large pan over a medium head and add a little olive oil. When hot, add the onions, carrots, tomatoes, garlic and orange zest and sweat gently for 5 minutes until softened and lightly coloured

Once the crabs are roasted, chop them roughly and add to the pan. Pour over enough cold water to cover everything and add the orange juice. Bring to a simmer and simmer for 1 hour, then strain through a sieve into another pan. Simmer to reduce by half, then take off the heat and allow to cool

You should be left with about 500ml of fish stock and 750ml of shore crab stock. Any you don't use for this recipe can be kept in the fridge for 2 days or frozen for up to a month

To make the Porthilly sauce, preheat a grill to high. Place the tomatoes on the grill rack and grill, turning occasionally, until they are soft and the skins are blistered all over. Pour 500ml of fish stock and 750ml of shore crab stock into a large saucepan and add the tomatoes and butter. Bring to the boil over a medium heat and then let it bubble to reduce down to about 400ml

Tip the contents of the pan into a blender and blitz for 4 minutes until smooth. Keep the sauce warm (or allow to cool if making ahead and reheat to serve)

For the gurnard, preheat an oven to 240°C/gas mark 9. Trim the edges of the gurnard fillets to neaten and season them with salt. Heat a large non-stock ovenproof frying pan over a medium heat

When the pan is hot, add a drizzle of olive oil and carefully add the fish fillets skin-side down. Cook for 2 minutes, then transfer the pan to the oven and cook for a further 2–3 minutes. Remove the pan from the oven; the fish should still look a little translucent on the top

Carefully turn the fillets over and leave them to finish cooking in the residual heat for 30 seconds or so

While the fish finishes cooking, give the sauce a good whisk and pour it equally into 4 warmed bowls. Using a palette knife, carefully lift each fish fillet from the pan and place on top of the sauce. Finish with a generous drizzle of extra virgin olive oil and serve straight away