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Sauté the onion and garlic in a medium-hot pan with a dash of olive oil for 5 minutes until softened. Season, then add the pine nuts and fry for about 1 minute until golden. Add the spinach and wilt briefly in the pan, tossing to mix well. Remove from the heat and stir in the feta.
Lay the saddle of lamb open on a board, flesh side up. Season with salt and pepper and sprinkle over the sumac. Spoon the spinach mixture along the middle of the meat, using the fillets that run down the inside length of the meat to support the sides of the stuffing.
Roll the meat around the filling and tie at intervals with string. Season the outside of the lamb all over, then chill for at least 30 minutes or overnight to help firm it up and make it easier to brown.
Preheat the oven to 190°C/Gas 5.
Put a roasting tray on the hob and heat until hot. Add a glug of oil and fry the joint for 10 minutes until brown all over. Transfer to the preheated oven and cook for 45-55 minutes, depending on the weight of the lamb and how pink you like it. When cooked, set aside to rest.
Meanwhile, mix all the dressing ingredients together and add a little seasoning.
Serve the rested lamb hot or at room temperature, thickly sliced, with the dressing on the side.
HOW TO STUFF MEAT
The secret of any stuffing is to part-cook it first. Raw onions and raw garlic will take for ever to cook inside the meat. If you are not going to be roasting the meat immediately, you must let the stuffing cool before using it. When filling the saddle, put extra stuffing at the edge as some will inevitably squeeze out as you roll, and don’t tie it too tightly.