Remove the fat from the cavity of the chicken. Rinse the chicken under cold water and pat dry with paper towel. Chop the chicken Chinese-style (see Note). Put the chicken, ginger, spring onion and 3 litres (12 cups) water into a pot large enough to fit the chicken snugly, and bring to the boil. Reduce the heat to a low simmer and skim the stock well. Simmer for 30 minutes, skimming continually. Reduce the heat until the surface is barely moving and cook for 2 hours. Remove from the heat, strain through a muslin-lined sieve, discard the solids and strain again.