Soak the hocks in a large pan of cold water overnight.
The next day, change the water, bring to the boil, then discard the water and cover with fresh cold water. Add the sliced onion, the carrot, celery, garlic, bay leaves and thyme. Bring to a simmer, skimming carefully, and cook over a gentle heat for 2½ hours, replenishing the water so it always covers the hocks.
Rinse the le