Preheat the oven to 190°C/gas mark 5. Put the lamb on a board and make small slits all over the joint. Press some of the sliced garlic and ginger and a few cumin seeds into each. Rub the oil into the joint.
Put the onion wedges and carrots in a large roasting tin and toss with 2 tbsp of the flour. Sit the lamb on top and pour in the ginger beer and stock. Roast in the oven for 1 hour. Roast for a further 40 minutes for medium-pink lamb, 50 minutes for well done.
Remove the lamb from the tin, cover with foil and leave to rest for 20 minutes. Pour the roasting juices from the tin into a small pan over a medium heat and simmer for 5 minutes, until slightly reduced. Make a paste with the remaining flour and 4 tbsp water and whisk into the gravy until thickened.
Strain the gravy (you can keep the veg to make a soup). Carve the lamb and serve with the gravy, roast potatoes and vegetables of your choice.