Peel and finely slice the onions. Peel the garlic cloves and crush or finely grate it. Trim and scrub the carrots and chop them into small chunks. Pick the thyme leaves of their stalks (if the stalks are woody you can push them through a metal sieve - the leaves will gather in the sieve).
Pour 1 tbsp oil into a large pan and bring to a medium heat. Add the onions, garlic, carrots and thyme to the pan along with a generous pinch of salt and pepper. Fry for 5 mins, stirring now and then, till softened.
While the onions are cooking, thickly slice the portobello and souping mushrooms. Stir into the pan and continue to cook for a further 6-8 mins till glossy and caramelised.
Tip in the pearl barley and cover with the boiling water. Bring to the boil, then reduce to a simmer and pop a lid on the pan. Cook gently for 25 mins, till the barley is tender.
Meanwhile, trim and thinly slice the runner beans. Trim away any thick stalks from the spinach and roughly shred the leaves. When the barley has had 25 mins, fold the spinach and beans into the pot. simmer for a further 5 mins, till the spinach has wilted slightly and the beans are cooked.
Spoon the mushroom and barley stew into bowls and serve.