Put the shoulder steaks into a large, shallow, non-metallic dish. Put the lemon juice and 4 tablespoons of the vinegar into a bowl. Peel the garlic and crush it into the bowl, then add a generous sprinkling of black pepper. Stir the mixture and pour it over the shoulder steaks. Cover the dish and leave to marinate in the refrigerator for 8 hours or overnight, turning the meat over about half-way through.
Have a saucepan of water ready to which you have added 2-3 tablespoons of wine vinegar. Cut the knobbly peel off the celeriac, cut in half and then into thin slices, putting them straight into the saucepan. Bring to the boil and boil for 5 minutes, then drain and pat dry.
Drain the marinade from the meat and pat dry. Heat the olive oil and 15 g/½ oz of the butter in a large, heavy-based frying pan over a fairly low heat. Add the pork and fry gently for about 10 minutes on each side, until golden brown. Using a slotted spatula, transfer the pork to a shallow serving dish and keep warm, uncovered, in a low oven. Add the remaining butter to the frying pan and put back over a medium heat. When the butter has melted, add the celeriac slices and sauté for a few minutes, until browned on both sides. Arrange among the pork steaks and continue to keep warm. Don’t wash out the frying pan; just before serving, empty the cream into the pan, add the crushed green peppercorns and season with salt. Bring to the boil and bubble for a minute, then stir in a small squeeze of lemon juice, pour the sauce over the pork and celeriac and serve at once.