Chop the beef into equal-sized chunks, around 2cm across. Warm a large pan over a high heat for 1 min. Add 2 tsp olive oil and the beef. Fry for 5-8 mins, turning once or twice, till the beef is browned all over.
While the beef fries, peel and finely chop the onion. Peel and dice the carrot to match the onion. Trim the dry ends off the celery stick and finely slice it. Peel and crush the garlic.
Lift the beef out of the pan and set aside on a plate. Add the chopped onion, carrot and celery to the pan with 2 tbsp water. Season with a good pinch of salt and pepper. Fry for 5 mins, stirring once or twice, till the veg have softened and browned a little.
While the veg fries, halve the squash. Scoop the seeds out. Peel it. Chop it into cubes around 1cm across.
Add the garlic and squash to the pan. Slide the beef back in, with any juices from the plate. Add the pearl barley and thyme sprigs and crumble in the vegetable stock cube.
Pour in 800ml boiling water. Cover, turn the heat up and bring to the boil. Turn the heat down and simmer for 40 mins till the meat is tender.
Lift the thyme sprigs out of the stew. Taste it and add more salt or pepper if you think it needs it. Ladle the stew into two warm bowls and serve straight away.