8 garlic cloves, thickly sliced
1 lemon, zest only
1 onion, cut into wedges
2 rosemary sprigs, leaves picked
sea salt and freshly ground black pepper
2 tsp cornflour
2 tbsp olive oil
40g/1陆oz unsalted butter
2kg/4lb 8oz leg of lamb, bone in
350ml/12fl oz Madeira