Remove the membrane on the back of the ribs by flipping the ribs so they are fleshy-side down and peeling off the thin layer of sinew on the back. This is easiest when they are cold so you can prize under the sinew with a cutlery knife if necessary.
Put the ribs into a shallow roasting tray and smother with the mustard, rubbing it all over the racks. (It’s important not to have the ribs overlapping because if you do, rather than being crispy, caramelised and deep golden brown when cooked, they will be flabby and a bit nasty.)
In a bowl, mix together the spices, sugar and seasoning. Rub the spice mix all over the racks. At this point it’s brilliant if you can leave the rub to do its thing. Keep the ribs marinating in the fridge for up to 48 hours and the flavour really seeps into the meat, but 30 minutes minimum will do if you’re pressed for time.
Preheat the oven to 150ºC/Fan 130ºC/Gas 2. Pour the apple juice down the side of the roasting tray, not over the ribs otherwise you’ll wash off the spices. Cover with foil.
Slow cook the ribs in the oven for 4 hours. Then, 30 minutes before the ribs are done, spread coals evenly over the basin of a barbecue and heat for 35–40 minutes until white hot. Remove the foil and cook the ribs straight on the hot barbecue for 5 minutes a side, letting the sugar become crispy and caramelly. If cooking this recipe in the oven, 30 minutes before the ribs are finished slow cooking, increase the heat to 220ºC/Fan 200ºC/Gas 7 and remove the foil. Cook for 10 minutes uncovered, allowing the top to crisp up nicely. Serve with slaw, baked potatoes and soured cream.