1. Fill your kettle and boil it. Tip the dried porcini mushrooms into a heatproof jug and pour over enough boiling water to cover them. Soak for 5 mins, then drain and rinse under cold water. Pop them back in the jug and cover them with 200ml boiling water. Set aside.
2. Pour any remaining hot water from the kettle into a medium pan, topping it up with extra if needed. Add a pinch of salt, cover and bring to the boil. When the water is boiling, tip in the pearl barley. Simmer for 20-25 mins, lid half-on the pan, till the barley is tender but still has a little chew. Keep your eye on the pan and if it looks like it's drying out, add a splash more water.
3. Meanwhile, peel and finely chop the onion. Set a deep frying pan or wok on a medium-low heat. Add the butter and swirl it round the pan to melt. Add the onion, season with salt and pepper and fry for 8-10 mins, stirring often, till the onion is soft and golden brown. If it starts to colour too quickly, turn the heat down a little.
4. While the onion fries, rinse all the fresh mushrooms. Tear the wonky and portobello mushrooms into bite-sized pieces.
5. Add all the fresh mushrooms to the onion and fry, stirring often, for 8-10 mins till juicy. Stir in 1 tbsp Dijon mustard, along with the porcini and their stock. Cover and simmer for 5 mins.
6. While the mushrooms simmer, roughly chop the parsley, discarding any really thick stalks (see our tips on how to use the leftover stalks).
7. Turn the heat under the mushroom pan down to very low. Add half the pot of soured cream to the pan and stir. Taste and add a pinch more salt or pepper if you think it needs it.
8. Drain the barley and divide it between 2 plates. Top with the mushroom Stroganoff and garnish with parsley to serve.