Place all the bun ingredients (except the butter) in a bread machine, along with 330g of water and 5g of sea salt.
Use a dough hook to combine, then leave the dough to prove for 20 minutes at room temperature. Return the mixture to the machine and add the butter. Use the dough hook to bring everything together. Weigh out the dough into 50g balls and leave to prove for 1 hour.
Peel and finely chop the onions, then pick and roughly chop the thyme. Finely chop the pickled walnuts.
Place the onions in a pan with a little oil and sweat down over a low heat for 5 minutes, until soft and sweet but not coloured.
Preheat the oven to 100ºC/gas ¼.
In a cast iron pan or casserole, fry the beef for 10 minutes over a medium-high heat, until it has lots of colour.
Deglaze the pan with the red wine. Add the rest of the filling ingredients (except the walnut juice), bring back to the boil, pop the lid on and slow cook in the oven for 4 hours.
Season to taste and add the pickled walnut juice. Your beef mixture should be quite moist at this stage, but if it is too loose, remove the lid and reduce it further over a high heat. Leave to cool, divide into golf-ball sized pieces and roll into balls.
Preheat the oven to 170ºC/325ºF/gas 3.
Flatten each dough ball, add a ball of beef and mould the dough around it. Place on a greased and lined baking tray, crease-side down, and bake for 15 minutes, until they rise slightly and turn golden. Best eaten warm.