Bring water to boil in a large pot. Rinse pork knuckle in cold water and add to the boiling water. Add quartered onions, bay leaves, peppercorns, juniper berries and sugar. Simmer over low heat in covered pot for 1-1/2 hours.
Remove pork knuckle and keep warm. Strain stock with a sieve. Return pork knuckle to pot and add 2 cups stock. Add whole onions, cloves, bay leaves and remaining sugar and simmer in covered pot for further 40 minutes.
Simmer sauerkraut for about 10 - 15 minutes while adding a little bit of stock. Arrange sauerkraut and pork knuckle on plate.