Rub the pork rib on all sides with dry mixture, let it sit for 15 minutes.
In the meantime, heat an uncovered deep roasting pan (just a little bit bigger than the rib roast) in the oven preheated to 250°C
Rub the meat with olive oil, place in a hot pan and bake, uncovered, for about 30 minutes until a golden-brown crust forms. Remove from the oven and from the baking pan, and reduce oven temperature to 150°C.
Spread apple and onion slices on the bottom of the pan, add cider.
Place the rib roast on apples and onions, tightly cover with a lid and bake for about 2 hours. Check, once or twice that there is enough liquid to prevent apples and onions from burning.
When done, let the rib roast rest for 15 minutes, then gently pull apart with a fork (or fingers) from the bone and serve with the apple and onion sauce and mashed potatoes.