Preheat the oven to 425 degrees F. Put the salt and caraway seeds into a bowl, mince or grate in the garlic, and stir to combine. Add the pork knuckles and rub them well with the caraway mixture, getting it into the slits in the rind where it was scored.
Peel the onions and slice them into rounds and add them to the bottom of a roasting tin, making a bed or platform. Sit the knuckles on top of the onions and cook them for 30 minutes.
Take the tin out of the oven and quickly arrange the apples and potatoes around the knuckles. Carefully pour 1 cup of the beer over the knuckles, so they are basted as the liquid pours into the tin. Put the pan back into the oven. Lower the oven temperature to 325 degrees F and roast for 2 hours.
Turn the oven up again to 425 degrees F, and baste the hocks with the remaining beer. Roast for 30 minutes more.
Remove the tin from the oven and transfer the apples and potatoes to a warmed serving dish. Lift the knuckles onto a carving board and leave the onion and juices in the tin. Put the tin on the stove over medium heat and add 2 cups boiling water, stirring to deglaze the pan to make a gravy. Transfer the gravy to a serving bowl.
Take the crackling off the knuckles and break it into pieces, then add them to a serving bowl. Pull or carve the pork meat and add it to the bowl with the crackling. Serve with the apples, potatoes, gravy and some German mustard.
Make Ahead Note:
The pork and onions can be put in roasting tin up to 1 day ahead. Cover with plastic wrap and refrigerate. Just before cooking, rub with the salt, caraway and garlic and cook as directed.
Making leftovers right:
Leftover pork can be stored in the refrigerator, tightly wrapped in aluminum foil, for up to 3 days. Eat it cold or reheat gently in a saucepan with leftover gravy, until piping hot. You should store any leftover gravy in a separate airtight container in the in the refrigerator for 1 to 2 days.
Leftover pork can be frozen for up to 2 months, tightly wrapped in foil, then defrosted overnight in the refrigerator. Even if you have an amount too small to be useful by itself (a likely outcome), simply bag and mark it up and freeze it for future use in the Pantry Paella recipe.