Melt half the butter in a large lidded frying pan. Cook the leeks over a medium heat, covered, for about 10 mins until soft, stirring occasionally so they don't catch.
Meanwhile, put the celeriac in a pan, cover with salted water, bring to the boil and cook for 15-20 mins until soft. Drain and roughly mash, then whizz with a stick blender to make an almost smooth mash. Stir in the leeks and the rest of the butter, season well and keep warm.
Put the apples and cider in a pan. Heat for 10 mins on a medium heat, stirring until the apples have broken down, then stir in the mustard, some seasoning and the sugar.
Heat the oil in the pan you cooked the leeks in and fry the gammon steaks on a medium-high heat for 3-4 mins each side. Serve with the mash and apple sauce on the side.