Heat the oven to 200C (180C fan/390F/gas 6. Pick off and discard the dried chilli stems, then soak the chillies in warm water for 20 minutes. Meanwhile, put the onion and garlic in a large roasting tray big enough to hold the lamb later, roast for 15 minutes, then add the tomatoes and roast for 10 minutes more. Remove from the oven, and turn up the heat to 220C (200C fan/425F/gas 7. Tip the onion mix into a blender with the tamarind and drained chillies, and blitz until semi-smooth. Scrape the mix into a large saucepan, add the stock, bring to a boil and reduce slightly for five minutes.
Season the lamb shoulder and roast in the same tray for 15-20 minutes, just until the meat starts to colour. Turn down the oven to 170C (150C fan)/325F/gas 3, and pour the adobo sauce over and around the lamb. Cover the tray tightly with foil and roast for another three hours, basting the meat in the sauce from time to time, until it’s nice and soft and the bone pulls away easily.
When the lamb is cooked, remove from the oven and leave to rest. The sauce will by now be infused with all the flavours of the roasting juices, but if it looks a little thin, reduce it a bit on the hob. Spread most of the adobo on a serving plate, lay the lamb on top, then pour over the rest of the sauce.