Preheat oven to 400°.
Rinse the meat and pat dry. Cut the fatty skin on the knuckles diagonally, in both directions, to create a cross-cut pattern. Sprinkle with salt, pepper, paprika and caraway seeds.
Heat shortening in a Dutch oven over high heat and brown the meat on all sides. Remove meat to a plate and remove the Dutch oven from the heat. Add the onion, celery, potatoes, carrots, juniper berries, bay leaf, cloves and mustard seeds. Place knuckles on top. Pour in the water, and place the pot in the middle of oven. Cook, uncovered, for 1½ hours. Baste with half of the beer from time to time, and add a little water if it becomes dry.
Remove from the oven and arrange the meat and vegetables on a platter. Deglaze the pot using the remaining beer and additional water, if needed, scraping the bottom well. Stir the cornstarch or flour into the water to dissolve, and stir in the tomato puree. Season with additional salt and pepper, if desired. Serve gravy over meat and vegetables, and garnish with parsley.