slow-roasted beef with mustard potatoes

serves 6, takes 3½ hrs

3 leeks (about 500g), trimmed, halved lengthways, then quartered
4 carrots (about 300g), trimmed and halved lengthways
2 tbsp Dijon mustard
2 tbsp light brown sugar
1.4kg beef roasting joint (we used topside)
½ beef stock cube, made up to 250ml
1.2kg potatoes, peeled and halved, or quartered if large
1 tbsp plain flour
2 tbsp English mustard powder
10g thyme sprigs plus extra to serve
1½ tbsp olive oil

Preheat the oven to gas 6, 200°C, fan 180°C.

Put the leeks and carrots in a large roasting tin. Mix the Dijon mustard, sugar and some seasoning in a bowl and spread all over the beef joint. Lay the beef on top of the veg. Pour the stock around the veg and cover with foil.

Roast for 30 mins, then reduce the oven to gas 3, 170°C, fan 150°C and roast for a further 2½ hrs, turning the meat halfway through.

After 1 hr at the lower temperature, put the potatoes in a large saucepan of water and bring to the boil. Simmer for 10 mins, then drain and leave to steam in the colander for 2-3 mins. Return to the pan and sprinkle with the flour, mustard powder and some seasoning. Shake to coat.

Lay the thyme over the base of a roasting tin and drizzle with half the oil. Put the potatoes on top and drizzle with the remaining oil. Roast on the lower shelf of the oven for 1½ hrs, shaking to turn 3 times during cooking.

Meanwhile, once the beef is done, remove from the oven and set aside to rest, still covered with foil, for 20 mins.

Cut half the beef into 12 thin slices, reserving the rest. Serve the beef with the vegetables, roast potatoes and extra thyme, with the pan juices alongside.