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Stuff the pork up to a few hours ahead; chill. Return to room temperature before roasting.
Preheat the oven to 220°C, fan 200°C, gas 7. Heat 1 tablespoon of the oil in a frying pan; add the leeks and cook for 8-10 minutes until soft; transfer to a bowl. Squeeze the sausagemeat from the skins and add to the leeks. Add the sage, breadcrumbs, almonds, half the fennel seeds and the lemon zest. Season and mix well.
Remove any string from the pork and unroll, skin-side down, on a clean board. Using a sharp knife, cut into the meat horizontally about two-thirds of the way along (but not all the way through), keeping your knife parallel to the board, to make a pocket. Open the meat out like a book and spread the stuffing into the opened-out joint, leaving a 2cm border. Roll up tightly; tie with string.
Weigh to calculate the cooking time (see step 4); transfer to a roasting tin. Rub the skin with 1 teaspoon of the oil. Mix the remaining fennel seeds with the sea salt and spread over the scored skin. Roast in the preheated oven for 20 minutes.
Meanwhile, in a bowl, toss the fennel with the remaining olive oil and the pancetta; season with black pepper. After 20 minutes, remove the pork from the oven and scatter the fennel mixture into the tin. Reduce the temperature to 160°C, fan 140°C, gas 3; return to the oven for 1 hour 30 minutes (or 25 minutes per 500g) until cooked through. Transfer the pork to a board, cover and leave to rest for at least 20 minutes.
Meanwhile, cook the kale in boiling water for 3-4 minutes until tender; drain and add to the tin with the fennel. Toss together, add the fennel fronds and lemon juice and season. Serve with the pork.