Put a large pan of salted water on to boil. Heat your grill to high.
Trim the roots and top 3cm off the leek. Halve it, rinse out any grit and then finely slice it. Warm a medium pan over a low heat for 2 mins, then add the leek and 1/2 tbsp oil. Season with a pinch of salt and pepper. Gently cook for 8-10 mins, stirring, till the leek is soft and lightly browned.
While the leek sweats, coarsely grate half the cheddar. Beat the egg, mustard and 1 tbsp of the vinegar (keep the rest for later) together in a bowl till well combined. Stir in the cheddar.
Line a grill pan or baking tray with foil. Lay the gammon steaks on a board and bash them with a rolling pin or a pestle to flatten them out a little, so they are around 1-2cm thick. Pop them on the grill pan or baking tray and slide under the grill. Grill for 10-15 mins, till they are lightly browned.
While the gammon grills, scrub the potatoes and chop into bite-size chunks. The water should be boiling by now, so add the potatoes and simmer for 15-20 mins.
While the potatoes simmer and when the leeks are soft and caramelised, take them off the heat and stir in the cheese and egg mixture. Season with a crack of ground pepper and mix well.
The gammon should be browned by now. Take out from under the grill and use a spatula to flip them over. Spoon the cheese and leek mixture over the top of each gammon steak. Return to the grill and cook for another 8 mins till the cheese and leek topping is golden brown and bubbling.
Whisk the remaining vinegar with 1 tbsp oil and a pinch of salt and pepper to make a dressing. Toss with the salad leaves.
The potatoes should be tender and cooked. Drain them well and then serve them with the golden cheese and leek gammon grills and the salad.