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Preheat the oven to 230°C, fan 210°C, gas 8. If there are still bones down the middle of the mackerel fillets, the quickest way to remove them is to cut a channel in the shape of a V, and pull the bones and a little of the flesh away. Season the fleshy side with salt and black pepper and leave to one side.
Put the tomatoes in a roasting tin or baking dish that’s big enough to hold the mackerel fillets later. Cut 1½ lemons into thin slices and lay in the base of the tin. Cut the tough tops off the fennel bulb and add those too, plus 1 tbsp olive oil. Roast on the top shelf of the oven for 5 minutes.
Meanwhile, use a mandoline, a food processor, or a very sharp knife to cut the fennel into wafer thin shavings, cutting lengthways from base to top. Put these in a large bowl with a pinch of salt; toss well. Set aside for 5 minutes.
Remove the roasting tin of tomatoes from the oven. Pour in the Pernod and lay the mackerel fillets on top, skin-side up. Brush the fish with a little more oil; return to the oven for 12 minutes.
Add 1 tablespoon of olive oil to the bowl of fennel, plus the juice of the remaining half lemon, some black pepper, the watercress and tarragon. Mix well, then divide between 2 plates.
Remove the roasting tin from the oven; divide the mackerel and tomatoes onto the 2 plates (discard the lemon slices and fennel tops). Pour the juices into a bowl and whisk in the butter with a fork or balloon whisk; season.
Pour the juices over the fish to serve.