1: Place the ham hock in a large saucepan and cover with cold water. Place on the hob and bring to the boil, then drain and cover with fresh water. Tie the peppercorns, coriander seeds and fennel seeds together in a muslin bag and place the bag in the pan. Add the onion, garlic, carrots and celery and bring to the boil. Turn the heat down to a simmer and cook gently for 21/2 hours until the ham hock is cooked – keep an eye on it and if the liquid looks a little low, add a splash of boiling water from the kettle. Leave to cool in the stock.
2: When the ham hock is cool, pick the ham from the bone and remove any sinew and skin (don’t discard the stock; keep it for making pea and ham soup). Flake and finely shred the meat. Warm a little oil in a frying pan over a medium–high heat and fry the ham until crisp. Drain on some kitchen paper and leave to cool.
3: In a stand mixer with a beater attachment, slowly soften the butter. Add the shallots, egg yolks, mustard, vinegar, mustard seeds, parsley, chervil, curry powder and cayenne pepper. When it’s all smooth and bound together, add the crispy ham. Season.
4: Transfer the butter to a small terrine or serving dish, then put in the fridge for at least 1 hour before you serve it.
5: Let the butter come to room temperature, toast the sourdough then spread the softened butter on top. Serve immediately.