Heat your oven to 180°C/160°C/Gas 5. Slice the tomatoes into 6-8 wedges each and pop in a roasting tin. Add the chilli (keep it whole, or halve and scoop out the seeds and white pith) and drizzle over 1 tbsp olive oil and a pinch of salt and pepper. Slide into the oven and roast for 20 mins.
Meanwhile, peel the red onion and finely chop it. Peel the garlic and crush or finely chop it.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the chopped onion and garlic. Sprinkle over 2 tsp oregano and fry for 4-5 mins, stirring frequently, till the onion is softened.
Pour the barley into the pan and crumble in the stock cube. Cover with 800ml boiling water and stir well. Bring to the boil, then reduce to a simmer. Pop a lid on the pan and let the barley bubble gently for 20-25 mins, till the grains are tender.
Once the tomatoes have roasted, remove the tin from the oven. Trim the stalk off the chilli and finely chop the flesh. Slide the chopped chilli into the pan along with the roast tomatoes. Use a stick blender to whizz a little, leaving things still quite chunky. No stick blender? Simply pour 1/3 of the soup into a blender and blitz, before stirring back into the rest of the soup.
Taste the soup and add more salt and pepper if needed. Ladle into bowls and serve.