Place the sauerkraut in a colander and rinse briefly to remove some of the salt from the brine; don't rinse it too much, or you will lose a lot of the flavor. (Alternatively, if the sauerkraut is not excessively salty, use as is.) Press to release most of the excess liquid and set aside. In a large nonreactive skillet, melt 3 tablespoons of the butter over medium-low heat and add the pancetta. Cook for 5 minutes; don't let the pancetta brown. Add the onions and continue to cook until they are soft but not browned, about 8 to 10 minutes. Transfer the bacon-onion mixture to a 3 1/2 or 4-quart non-reactive casserole or ovenproof Dutch oven. Add the drained sauerkraut and toss to combine. Using a small piece of cheesecloth, make a bouquet garni with the thyme, bay leaves, peppercorns, juniper berries, and garlic and place in the baking dish. Add the ham hocks, chicken stock, and beer and stir to combine. Cover the casserole and bake, undisturbed, for 2 hours.
Meanwhile, melt the remaining tablespoon of butter in a large skillet over high heat and brown the sausages on both sides. Set aside.
Place the new potatoes in a saucepan and add water to cover. Bring to a boil, reduce the heat to a simmer, and cook until the potatoes are just tender, about 15 minutes. (This will depend on the size of your potatoes.) Drain and set aside.
When the sauerkraut and ham hocks have baked for 2 hours and the hocks are tender, remove the casserole from the oven. Place the sausages and potatoes on top of the sauerkraut. If the liquid has reduced to less than 2/3, add a bit more water. Cover the casserole and return it to the oven. Cook for about 30 minutes, or until the potatoes are very tender and the sausages are heated through. Remove the casserole from the oven and discard the bouquet garni. Serve immediately, with each person receiving some of each of the sausages, part of a hock, some potatoes, and sauerkraut. Pass the mustard at the table.