This has a couple of phases. You essentially make and reduce a spiced lamb
ragù, then add some things and wrap it up in pastry. For the bulk of the lamb filling, start by roughly chopping the onions and tomatoes, and peeling the garlic. Quarters will do, really, for the veg. Chop the coriander stalks, and some of the leaves.Sear the lamb shoulder in a heavy, lidded casserole, over a high-ish heat. There's no need to take it off the bone unless it won't fit into the pan. Get a bit of colour on all sides, then remove it, lower the heat, and add the onions. Fry them for a few minutes, softening and picking up a bit of colour, then add the tomatoes and garlic. Cook everything for another couple of minutes, making sure to work any sticky tasty bits off the base of the pan with a wooden spoon.
Add the stock, ensure it's all well combined, and carefully put the lamb back into the casserole. It should be just covered, but it's not massively important if there's a little bit poking out at the top. Add the oregano, cumin, coriander, and sumac, and give it a stir. Then put the lid on and stick it in the oven on around 160 for an hour and a half.Toast the nuts in a dry frying pan or skillet, and very roughly chop them.After the hour and a half, the lamb should be starting to fall apart, and come off the bone. If not, give it another half hour or so, probably with the lid off to reduce.Remove the lamb and leave it to cool somewhere for a while, and when it's cooled enough to handle, shred it off the bone and back into the sauce. On the hob, gently simmer the whole thing to reduce, stirring to break up any chunks of lamb - it should be thick, sticky, and shredded. Drive off most of the moisture (I'd guess twenty minutes, stirring occasionally), then take it off the heat and stir in the harissa paste and nuts, leaving it all to cool.You can stop here and finish the pie much later if you like, or just eat it as a rich stew if you reduce it a little less.
To finish, wilt down the spinach in a pan (or thaw, for frozen) and press out the moisture. Rough chop it when it's cool enough to handle. Chop the remaining coriander. Mix the spinach, breadcrumbs, coriander, and an egg into the cooled lamb mixture.Be careful with the coriander - it's easy to add too much.Now it's time for the pastry. Reserve about a third for the lid, and roll out the rest to roughly a half cm thickness, and a round shape a little bigger than a generous cake tin. Place this on a greased baking sheet, and roll out a lid of similar diameter - we're going to lay it on top of the filling to make a funky kind of flying saucer shaped pie. Beat the other egg and set it to one side.
Heap the filling onto the pastry base, in a round in the centre, keeping it a couple of centimeters from the edges. Brush the edge with egg wash and lay the lid over the top. Press the edges to seal, and trim it all into a round (cutting around a cake tin or basin helps).You can finish by crimping the edges with a fork, or score the top, if you like. Put a little hole into the very top to let steam escape, then brush it with egg wash and put it into a 180c ish oven for about half an hour, maybe a bit longer. Basically until it's fluffy and golden.