Heat the oven to 160°C/140°C fan/gas 3.Put the onions in the bottom of a large deep-sided casserole or roasting tray if you don’t have a casserole big enough to fit the leg of lamb in.
Sit the lamb on top of the onions. Blend together the garlic, rosemary, thyme, lemon zest, paprika, salt and olive oil to give a thick paste. Spread the paste all over the lamb and put into the oven for 45 minutes.
Pour the sherry over and the lamb and baste the lamb with the liquor in the bottom of the casserole or tray. Return to the oven and cook for a further 3 hours, basting the lamb every 45 minutes-1 hour. If the liquid is evaporating, add some water.
While the lamb is cooking, make the mint sauce. Dissolve the sugar in 3 tablespoons of boiling water. Mix in the cumin and vinegar. Just before serving add the chopped mint so it retains its bright green colour.
Remove the dish/tray from the oven and transfer the lamb to a board, ready to carve.
Pour off some of the lamb fat/oil from the cooking liquor, strain the liquor into a jug and serve with the lamb.