Combine the garlic, ginger, chilli, lemon juice, turmeric, cumin and coriander and salt with the yogurt.
Lay the onion disks in a roasting tray and sit the shoulder on top. Stab the lamb all over using a sharp knife and coat it in the spiced yogurt.
Cover with foil and put in an oven at 160 (140 fan) for 5-6 hours.
Uncover, turn the oven up to 160 fan and add the potatoes to the tray. Turn the potatoes over to coat them in the pan flavours. Sprinkle the Garam Masala over the potatoes and lamb.
Return the tray back to the oven for 50-60 minutes. The lamb should have taken on a lovely colour, charred in places and the potatoes should be golden.
Stir the spinach into the tray and serve.
