For the pastry, wrap the butter in foil and place in the freezer the night before you need it. Put the ham hocks, bay leaves and peppercorns in a large pan. Cover with water and a lid, and simmer for 3 hrs or until really tender – you may need to top up the water level during cooking. (Alternatively, cook in a slow cooker for 6-8 hrs.) Leave to cool in the liquid, then chill overnight if you have time.
To make the pastry, tip the flour into a big bowl with 1/2 tsp salt and the mustard powder. Hold the butter block in the foil (peeling back a little at a time as you need to) and coarsely grate straight into the bowl of flour, dipping the end of the butter into the flour every
so often – this helps to stop all the butter clumping together. Grate in the cheese too. Use a cutlery knife to lightly stir together, then stir in 175-200ml cold water to bring the dough together. Wrap in cling film and chill for 30 mins.
Meanwhile, cook the potatoes in a pan of boiling water for 8 mins or until just cooked, then drain. Melt the butter in another large pan and add the shallots. Cook gently for 10-15 mins until tender and golden, shaking the pan now and then to roll them around in the butter. While they cook, drain the ham hocks, reserving the cooking liquid – the meat should be nice and tender. Shred with two forks, discarding the fat and bones.
When the shallots are tender, scoop them out of the pan and add to the ham. Add the flour and mustard powder to the butter, stirring around the pan to make a paste. Cook for 1-2 mins, then splash in a little of the milk, stirring until smooth. Continue adding the milk, bit by bit, stirring well after each addition, until you have a very thick, smooth sauce. Add enough of the ham’s cooking liquid to thin the sauce to a double cream consistency, about 300ml. Taste and season with pepper – it should be salty enough from the ham stock, but add a little extra if it needs it. Stir in the ham, potatoes, shallots, wholegrain mustard, honey and parsley, then leave to cool.
Heat oven to 200C/180C fan/gas 6 and place a baking sheet on the middle shelf to heat up. Remove the pastry from the fridge. Divide into 2 balls, one a little larger than the other – keep the smaller one wrapped until later. On a lightly floured surface, roll out the large ball of dough to the thickness of a £1 coin. Use this to line the base of a 23cm metal pie plate or shallow cake tin. Fill with the pie filling, then brush the edges with a little beaten egg. Roll out the remaining pastry, lay over the top, then trim the excess. Crimp the sides of the pie, if you like. Brush with more egg wash and poke 2 steam holes in the top with a sharp knife. Place the pie on the hot baking sheet and bake for 45-50 mins until crisp and golden. Serve piping hot with green veg and gravy, if you like.