To make the spice mix, grind the caraway seeds, paprika, garlic, mustard seeds, nutmeg and salt using a mortar and pestle. Rub the spice mix into the knuckles, place them next to each other on a baking tray and cover with foil.
Roast for approx. 3 hrs. in the centre of an oven preheated to 150°C. Using two forks, loosen the meat from the bone and pull it away.
Thinly slice the red cabbage. Bring the water to the boil with the vinegar, salt and sugar. Add the red cabbage and allow to infuse for several minutes.
Cut the Gruyère into 4 slices. Spread the crème fraîche over half of the slices of bread, top with the meat and cheese, place on a baking tray and warm through in the oven for approx. 3-4 mins. at 150 °C until the cheese starts to melt, remove from the oven. Slice the radish, arrange on top of the sandwiches with the drained red cabbage, cover each one with a slice of bread, serve immediately.