Heat the oven to 220c/fan 200c/gas 7. On a board, strip the rosemary from its stems, combining the leaves with 1 tbsp salt and a heavy bombardment of black pepper. Chop this all together until you are left with a wonderful lichen-coloured rosemary-salt mix.
Take the lamb shoulder and, with the tip of a very sharp knife, lightly score the top side with close parallel lines, making sure you do not cut into the meat. Rub in your salt and rosemary mix, really massaging it into the scoring you have made, and not forgetting the underside of the joint. Leave the lamb to one side.
Skin the jerusalem artichokes, carrots and parsnips, splitting the parsnips and artichokes in half. Put them straight into salted boiling water and parboil them all for no more than 6 minutes. Cool immediately under a cold tap in a colander. Toss them in a bowl in a little olive oil, black pepper and salt.
On a baking tray lay the vegetables in their respective groups, putting the lamb directly on top of them all. Lightly pour a little olive oil over the top of the shoulder. Cook it for approximately 45 minutes in a preheated oven. The joint should be pink but not raw. Before serving, take a secret slice from a hidden part of the shoulder. (don’t wing it. Taking meat to the table only to remove it for further cooking is a disappointment to all.)
Meanwhile, peel and halve the onions, and finely dice. Melt the butter in a pan. Throw in the onion with some salt and a good grind of black pepper, and sweat for about 10 minutes. Pour in the wine, sprinkle in the sugar and cook until the onion is soft, about another 20 minutes.
Add the vinegar at this point, and keep on cooking until all the liquid has evaporated. The onions should not be coloured. Sprinkle over the flour and stir in thoroughly. Use a whisk as it is effective for eradicating lumps of flour.
Start adding the milk slowly, in dribs and drabs, stirring all the time. When all the milk has been used, cook the sauce very gently for a further 10 minutes. The consistency of the sauce should be that of runny double cream. If you think it’s too thick, thin it down with a little more milk. Season it. The onion sauce will be one step away from ready.
Remove the lamb and put it on a board to rest, sticking the vegetables back in the oven and cooking them for a further 10 minutes. Remove the vegetables and put on a platter. Put the oven tray on the hob, and pour in 50ml water, stirring vigorously to lift any catchments and juices. Pour this juice through a sieve into your onion sauce and stir in.
Slice the lamb and lay over the vegetables, taking the trouble to pour any additional juices from the joint into the sauce. Pour the sauce around the handsome sliced shoulder and roots. This is one heck of a lunch.