Preheat the oven to 100°C.
Sauté the chopped onions in a pan with oil until soft and sweet.
Brown the beef mince in a separate pan and deglaze with red wine.
Mix the meat, onions, and all other mince ingredients except the vinegar, then cook in the oven for four hours.
Mix the dough ingredients, excluding butter, in a mixer, then let prove in an oiled bowl for 20 minutes.
Return the dough to the mixer, incorporate the butter, then form 50g balls and prove for one hour on baking trays.
Stir walnut vinegar into the cooked mince and remove the star anise.
Increase oven temperature to 170°C.
Roll cooled mince into 30g balls, top each with a dough ball, press into domes, and bake for 15 minutes.
Whisk double cream to soft peaks, then fold in horseradish, lemon juice, and salt.
Quarter the pre-roasted Roscoff onions and char them in a hot non-stick pan.
Whisk oil, sliced walnuts, vinegar, and salt together for the salad dressing.
Toss watercress and charred onions with the dressing and serve with buns and horseradish cream.