Heat the oven to 200C/Gas Mark 6. Open out the saddle of lamb; set aside the eye fillets. Halve the garlic, season and leave for a few minutes to draw out the juices.
For the stuffing, heat a splash of olive oil in a non-stick frying pan and sauté the mushrooms until softened and browned; drain in a colander. Heat a little more oil and a knob of butter in the pan, add the spinach, season lightly and cook until just wilted. Tip the mushrooms into a bowl, mix in the spinach, mascarpone and egg yolk, and season with nutmeg, salt and pepper.
Oil several sage leaves and place down the middle of the lamb. Rub the meat with the garlic bulb. Spoon the stuffing on top of the sage leaves. Cut the eye fillets horizontally (but not right through), open out and place over the stuffing. Fold one flap of the saddle over, then the other, wrapping firmly to make a neat roll. Secure with string at intervals and tuck the remaining sage leaves in at the ends.
For the lamb
Heat a splash of oil in a roasting pan, add the garlic bulb and a few sage sprigs and sauté briefly, then add the butter and allow to foam. Put the meat on top.
Roast for 30 minutes, then lower setting to 180C/Gas Mark 5 and roast for a further 30 minutes. Lift meat onto a warm platter and rest in a warm place for 20 minutes. Deglaze pan juices with sherry vinegar, add the wine and stock and reduce by half. Strain and adjust seasoning. Serve the lamb thickly sliced with the jus.