Sauté the Base Vegetables: Heat the oil in a large, heavy-based casserole dish or Dutch oven over a medium heat. Add the chopped onions, carrots, parsnips, and celery. Cook for 10-12 minutes, stirring occasionally, until the vegetables have softened and are just starting to caramelise at the edges. This step is crucial for developing sweetness.

Add Aromatics and Mushrooms: Add the chopped garlic and the mushrooms to the pot. Cook for another 5-7 minutes, until the mushrooms have released their liquid and started to brown. Season generously with salt and pepper.

Create the Roux: Sprinkle the plain flour over the vegetables and stir continuously for 2 minutes. The flour will coat the vegetables and cook out its raw taste. Stir in the tomato purée and cook for another minute until it darkens slightly. I find this step really deepens the final flavour.

Deglaze and Build the Gravy: Pour in the stout, stirring and scraping the bottom of the pan to lift any caramelised bits – this is where so much flavour is hiding. Let it bubble away for a minute or two to reduce slightly.

Simmer the Stew: Gradually pour in the hot vegetable stock, stirring as you go to create a smooth gravy. Add the rinsed pearl barley, the potato chunks, bay leaves, thyme sprigs, Henderson's Relish, and the teaspoon of Marmite. Bring the stew to a gentle simmer.

Cook Low and Slow: Once simmering, reduce the heat to low, cover the pot with a lid, and let it cook for 50-60 minutes. Stir it every 15 minutes or so to prevent the barley from sticking. The stew is ready when the potatoes and root vegetables are tender and the gravy has thickened beautifully.

Final Touches: Remove the bay leaves and thyme sprigs. Taste the stew and adjust the seasoning with more salt and pepper if needed. Serve hot, scattered with a generous amount of fresh chopped parsley.