Preheat the oven to 160C/ 320F/ GM 2½
Chop the braising steak into small pieces (about 1.5cm cubes), toss in the flour
Melt ¼ of the butter in a casserole pot and fry ⅓ of the meat until brown all over. Remove from the pan, add in another ¼ of the butter and fry another ⅓ of the meat. Repeat with the remaining meat. Set it all aside
Add the final ¼ of butter to the pan and fry the leeks gently until soft (5-10 minutes). Stir often
Add the meat back into the pot along with any residual flour, the mustard, seasoning, 400ml of stock and a few sprigs of thyme. Cook for around 2 hours until the meat is tender, stirring halfway through
When the meat is almost cooked prepare the mashed potato: peel and chop the potatoes, then boil until soft. Mash, adding in butter and seasoning. Add a splash of milk if the mash appears too stiff to spread
Optional: remove the sprigs of thyme from the pie filling
Spoon the cooked meat into an ovenproof pie dish and top with the mash, spreading it out to cover the meat entirely. Use a fork to create texture or a pattern on the top of the mash
Sprinkle crumbled Blue Stilton cheese and breadcrumbs over the top
Turn up the oven to 200C/ 400F/ GM 6 and bake the pie for 30 minutes until golden