Start off by cutting one of the pork fillets almost in half lengthways (using a sharp knife) and open the 2 halves out.
Now do the same with the other fillet. Next, use a rolling pin to bash the fillets all the way down to flatten them a bit, they need to be about 5-6 inches (13-15 cm) wide, then season them with salt and freshly milled black pepper.
Now prepare the stuffing. Heat 1 oz (25 g) of the butter in a small saucepan, then gently fry the chopped onion for 10 minutes until softened, and add it (and its juices) to the bowl containing the remaining stuffing ingredients: the breadcrumbs, walnuts, apple, lemon zest and juice, egg and sage.
Now mix everything lightly together, taste and season, then pile half of the stuffing along the length of one half of the split fillets. Fold the other half of the fillet over the top, then tie with string to keep it in shape. Then do the same with the other fillet and the rest of the stuffing. Next, rub each fillet with 1 oz (25 g) of butter, place them in the roasting tin and bake for about 40-50 minutes, basting now and then.
When the fillets are cooked, remove them to a warmed serving dish and now make the sauce. Place the roasting tin over direct heat and stir in the flour. Mix, then allow this to brown for a couple of minutes before gradually adding in the cider and stock. Let it bubble for a bit and reduce for 3-4 minutes, then season and strain the sauce and serve immediately with the meat, cut into slices.
Pre-heat the oven to gas mark 5, 375°F (190°C).
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