Melt the butter in a large pan over a medium heat, add a splash of olive oil and the steak, and lightly brown the meat all over.
Stir in the Marmite and Worcestershire sauce, turn the heat up to high and keep stirring until all the liquid has evaporated.
Meanwhile, peel and/or trim and chop the onion, carrots and celery.
Add the chopped veg to the pan with the bay leaf and rosemary sprig and cook over a low heat with the lid on until softened.
Stir in the flour and, after 1 minute, pour in the stock. Season well with sea salt and black pepper.
Bring to the boil, then reduce to a simmer, add the pearl barley and cook gently for 1 hour, then remove from the heat and discard the rosemary sprig (if you like) and bay leaf.
Whiz the soup with a stick blender for a couple of seconds, so it thickens, but there are still some chunky bits, then give everything a good stir.