Make the terrine at least the day before you plan to serve it: place the ham hocks in a large pan, cover with water and bring to the boil. Lift the ham out the pan, discard the water and replace the ham in the pan. Cover with fresh water, add the onion and the carrots with the thyme, parsley stalks and peppercorns, and bring back to the boil. Skim off the scum and simmer