Dissolve the stock cubes in the boiling water.
Add all the remaining ingredients to the slow cooker (apart from the parsley, salt, and pepper). Pour over the stock.
Mix well, then cook on high for 4-5 hours.
When all the veg is cooked and the pearl barley is tender, remove a couple of ladles of veg and stock, then blitz it in a food processor (or use a stick blender) until it's smooth.
Add this back into the soup, and stir. This will thicken it slightly.
Season to taste with salt and pepper, then sprinkle over the parsley and serve!