Place the lamb in a roasting tin on top of the sliced onion. Insert thin slices of garlic and rosemary under the skin having made small incisions.
Pour a little oil over the lamb, salt & pepper, then add 1/2 glass water around the lamb.
Cook in the oven on a high shelf for 30 minutes. Baste and return to the oven for a further 15 minutes per 450g cooking time.
Check the meat is cooked by testing with a skewer. (There is no specific timing as some people prefer their lamb cooked through completely - this will require a longer cooking time.)
Remove from the oven, cover with foil or cloth and leave to stand for at least 30 minutes, whilst you make the gravy.
Pour off the fat from the roasting tin. Place the roasting tin on a direct moderate heat and, with a metal spoon, scrape the sides and base to release the crusty bits. (These provide flavour and colour.)
Add the flour and, with a wooden spoon or spatula, mix together to a roux. Cook for a moment or two and then gradually add stock or vegetable water to the gravy until it has reached the desired consistency.
Always make sure the gravy simmers before finally adjusting the thickness and seasoning etc.