1 Preheat the oven to 150°C/130˚C fan/gas mark 2.
2 Heat one tablespoon of the oil in a heavy-bottomed ovenproof pot over a medium-high heat. Brown the beef on all sides, then remove to a plate.
3 Add the butter. Add the onions, sprinkle with a little salt and pepper and cook for 6-8 minutes, stirring often, until lightly browned. Add the garlic and chillies and cook for one minute longer.
4 Keeping the onion mixture in the pot, place the beef on top. Add the stock and coriander stems; the stock should come about three-quarters of the way up the beef, so add more if necessary.
5 Bring to a boil, then cover the pot and place in the oven for two hours, turning the beef halfway through.
6 Add the tinned tomatoes and the butternut squash. Cover the pot and cook for one hour longer or until the beef is very tender.
7 Remove the pot from the oven. Transfer the beef to a cutting board and cover loosely with tin foil. Rest for 10 minutes.
8 Return the pot to the hob. In a cup, combine the cornflour with three tablespoons of water and mix into a slurry. Add this to the pot and stir for five minutes to thicken. Allow to bubble over a medium heat for 10-15 minutes until slightly reduced.
9 Slice the beef against the grain and add back to the onions and squash. Scatter with the coriander leaves, then serve with potato wedges and sour cream on the side.