In a small bowl, mix together the mustard and the mayonnaise, then set aside.
Mix the ham hock, ketchup, honey and 1 tbsp of vinegar together in another bowl, and pop in the fridge while you make the slaw.
Tip the fennel, apple and cabbage into a large bowl, add the remaining vinegar, stir and season well.
Assemble the burgers by spreading the mustard mayonnaise on the cut sides of the buns, pile on the ham hock, then the slaw. Serve immediately with the rest of the slaw on the side. Or pop the leftover slaw in the fridge for the next day.