For the beef mince, preheat the oven to 100C/220F/Gas ¼. Heat a dash of oil in a pan and fry the finely chopped onions until softened and sweet, but not browned.
Fry the beef mince in a separate pan in oil until golden-brown in colour. Add the red wine to deglaze the pan.
Stir in the cooked onions and all the other ingredients, except the pickled walnut vinegar. Cook in the oven for four hours.
For the dough, combine all the ingredients together except the butter in a free-standing mixer fitted with a dough hook. Tip into an oiled bowl, cover with cling film and leave to prove for 20 minutes at room temperature.
Place the mixture back in the machine and incorporate the butter for two minutes until fully mixed. Weigh out the dough into 50g/1¾oz balls, place on baking trays, cover with oiled cling film and leave to prove again for one hour.
Remove the mince from the oven after four hours and add the pickled walnut juice. Remove the star anise.
Increase the oven temperature to 170C/340F/Gas 3.
Leave the mince to cool and roll into little 30g/1oz balls. Place a ball of dough on top of each ball of mince. Push down to incorporate the meat mixture into the dough to form domes. Bake in the oven for 15 minutes.
For the horseradish cream, whisk the cream in a bowl until soft peaks form when the whisk is removed. Stir the horseradish into the whipped cream and add lemon juice and salt. Set aside.
For the watercress salad with pickled walnuts, peel the onions and leave to cool. Keeping the root on, cut each onion into quarters.
In a very hot non-stick pan, char the quartered onions until brown on each side.
Combine the oil, sliced walnuts, vinegar and a pinch of salt in a small bowl. Wash and drain the watercress.
In a salad bowl, toss the watercress and the onions and combine well with the walnut dressing. Serve with the horseradish cream and beef buns.