Preheat the oven to 140C/ gas mark 1.
On the hob, brown the lamb, fat side down, in a large roasting tin. Remove when nicely browned in the middle (you won't get much more than this) and set aside while you fry the vegetables briefly. Just tip them into the pan - you won't need to add any more fat - and cook them, sprinkled with salt, gently for a couple of minutes. Pour the water over and then replace the lamb, this time fat side up. Let the liquid in the pan come to a bubble, then tent with foil and put in the preheated oven.
Now just leave it there while you sleep. I find that if I put the lamb in before I go to bed, it's perfect by lunchtime the next day. But the point is, at this temperature, nothing's going to go wrong with the lamb if you cook it for a little less or a little more.
If you want to cook the lamb the day you're going to eat it, heat the oven to 170C/ gas mark 3 and give it 5 hours or so. The point is to find a way of cooking that suits you.
About an hour before you want to eat, remove the lamb from the tin to a large plate or carving board; not that it needs carving: the deal here is that it's unfashionably overcooked, falling to tender shreds at the touch of a fork. This is the best way to deal with shoulder of lamb: it's cheaper than leg, and the flavour is deeper, better, truer, but even good carvers, which I most definitely am not, can get unstuck trying to slice it.
I get on with the vegetables while the lamb's sitting meekly, but you could equally well have done this earlier. But to finish the lamb salad, simply pull it to pieces with a couple forks on a large plate. Sprinkle with more salt and some freshly chopped mint, then cut the pomegranate in half and dot with the seeds from one of the halves. This is easily done; there's a simple trick, which means you never have to think of winkling out the jewelled pips with a safety pin ever again. Simply hold the pomegranate half above the plate, take a wooden spoon and start bashing the curved skin side with it. Nothing will happen for a few seconds, but have faith. In a short while the red glassy, juicy beads will start raining down.
Take the other half and squeeze the preposterously pink juices over the warm shredded meat. Take to the table and serve.