In a crock pot, combine onion rings, soy sauce, and brown sugar. Wisk beer, mustard, and caraway seeds together. Mix into onion rings in crock pot. Place brats in crockpot and mix all well. Cover and cook brats 6 hours on low or 3 hours on high. Take brats out of crock pot and keep warm. Stir sauerkraut into crockpot and let sit about 5 minutes, until warm. Strain onion/sauerkraut mixture from the crock pot to use as brat topping.