Slow roast shoulder of lamb with sliced potatoes

Name: Slow roast shoulder of lamb with sliced potatoes
Ingredients: 2.25kg/5lb whole lamb shoulder, skin on; 4 garlic cloves, sliced; 2 large onions, cut into quarters; 750g/1lb 10oz waxy potatoes, such as Desiree, peeled and cut into thick 2cm/¾in slices; 570ml/1 pint hot lamb or beef stock; green vegetables, to serve; 3 tbsp lamb fat (see method); 50g/1¾oz plain flour; 450ml/16fl oz hot lamb or beef stock; 100ml/3½fl oz red wine; 1–2 tsp redcurrant jelly; few drops Worcestershire sauce; gravy browning (optional)