Begin by making about 24 small, deep cuts in the skin of the lamb using a small, sharp knife.
Then push a sliver of garlic, followed by a small sprig of rosemary, into each cut, and season the meat generously with salt and freshly milled black pepper.
Next, cut the onion in half and place it in the bottom of the roasting tin, then transfer the lamb to the tin to sit on top of the onion halves.
Cover the tin loosely with foil, then cook in the oven on a high shelf for 1½ hours. After this, take the foil off and let it cook for another 30 minutes.
Remove the lamb from the oven, cover loosely with foil again and allow to rest for about 20 minutes.
Meanwhile, make the Rosemary and Onion Sauce.
In a small saucepan, melt the butter and cook the onions over a very gentle heat for about 5 minutes – it’s important not to let them colour, so keep an eye on them.
While that’s happening, bruise the rosemary leaves with a pestle and mortar to release their oil, then chop them very, very finely and add them to the onion.
Then continue to cook as gently as possible for a further 15 minutes, again, without letting the onions colour too much.
Next, using a wooden spoon, stir the flour into the onions and their buttery juices till smooth, then gradually add the milk, a little at a time, still stirring, followed by the stock, bit by bit, whilst vigorously whisking with a balloon whisk.
Now taste and season the sauce with salt and pepper and let it barely simmer on the lowest possible heat for 5 minutes.
Next, remove it from the heat, then liquidise or process half of it, then return it to the saucepan to join the other half.
Then re-heat gently, add the crème fraîche and pour it into a warmed serving jug.
This recipe makes about 1 pint (570 ml) sauce.