Place hocks, water and onion in a large stockpot and simmer over low heat eight hours. Add more water as needed to yield one gallon of final product. Strain and place stock in refrigerator overnight. Using a large spoon, remove fat layer from top of chilled stock. Stock should be slightly gelatinous. Stock can be frozen in small batches.
Yield: one gallon
Note: Reserve ham hock meat for other recipes