Heat your oven to 200°C/Fan 180°C/Gas 6. Halve the spaghetti squash. Scoop out the seeded middle bit with a spoon. Place the squash halves on a baking tray, cut-side up. Drizzle with ½ tbsp. oil. Roast in the oven for 30 mins till just soft.
Cut the tomatoes into wedges and scatter on a baking tray, along with the olives, ½-1 tsp chilli flakes (or more if you like things spicy), 1 tsp dried oregano, 1 tsp oil and some salt and pepper. Toss together, then bake for 20 mins on the shelf below the squash.
While the veg roast, tip the barley into a sieve and give it a quick rinse. Tip into a small pan. Pour in enough boiling water to cover by 3cm. Pop on a lid. Bring to the boil, then turn down the heat and simmer for 20 mins till the barley is soft.
Take the squash out of the oven and set aside to cool. Tip the tomatoes and olives into a food processor. Add 1 tbsp balsamic vinegar and blitz to make a sauce. No food processor? Tip into a bowl and use a potato masher to crush everything together. Taste and add more salt and pepper if you think it needs it.
Using your hands, peel the skin from the cooled squash (see our tip). Pull the squash’s flesh apart with two forks to create ‘spaghetti’. Spread the squash spaghetti back out on the baking tray.
Pour the tomato and olive sauce over the squash spaghetti. Toss together till well coated. Cover with a layer of greaseproof paper. Put back in the oven for a further 10 mins to warm the spaghetti through.
The pearl barley should be cooked by now, so drain well. Spoon the barley into bowls. Top with the squash, sauce, and a few handfuls of rocket. Serve straight away.