STEP 1
Heat 2 tbsp oil in a large pan over a medium heat and add the onion. Cook until soft, then scoop out with a slotted spoon and put in a food processor. Add the mustard seeds to the pan and cook until they pop, then tip out into a bowl to cool.
STEP 2
Prepare a sink full of iced water. Bring a pan of salted water to the boil and blanch the kale for 1 minute, then drain and cool in the iced water. Drain, then squeeze as much liquid from the kale as possible. Add it to the food processor. Whizz the onion and kale until finely chopped, then stir in the mustard seeds and powder, cayenne and vinegar, and season – it should be quite spicy.
STEP 3
Remove the string from the gammon and unroll to a rectangle. Cut deep horizontal slits in the surface of the meat about 4cm apart, being careful not to go right through, and stuff with the kale mixture, forcing in as much as possible, then spread a layer on top (you may be able to get it all in). Roll the meat up and tie with butcher’s string at intervals.
STEP 4
Wrap the gammon in a double layer of cheesecloth and put in a large pan. Cover with cold water, bring to the boil, then simmer, partially covered, for 20 minutes per 450g, plus 20 minutes. Allow to cool completely in the liquid, then slice and serve straightaway or chill until needed.
STEP 5
If you want to glaze the ham, remove the cheesecloth and carefully cut away and remove the string. Heat the oven to 200C/fan 180C/gas 6. Use a sharp knife to cut away the skin leaving an even layer of fat. Mix the glaze ingredients and brush over the surface of the fat. Bake for 15-20 minutes until the fat is golden and glazed.