Heat oven to 160C/140C fan/gas 3. Pour the stock into a flameproof roasting tin or shallow casserole dish, add 3 tbsp mustard and bring to the boil. Turn off the heat, then scatter in all of the vegetables except the radishes and onions. Nestle the beef in the middle, fat-side up. Brush the top of the beef with the remaining mustard, scatter over 1 tsp cracked black pepper, cover with a lid or foil and put in the oven for to braise for 3 hrs, or until the beef is tender.
Leave everything to cool a little, then lift the meat from the dish, transfer to a board and carve into thin slices. Stir the onions through the broth with the other vegetables, then divide the braised vegetables between bowls and pour in the mustardy cooking juices. Drape the slices of beef over the top, then poke in some radishes so they’re in amongst it all. Scatter the dill over just before serving.