Heat the plant-based butter or oil in a large pan on medium heat. Add the onion and stir occasionally for around five minutes or until it starts to soften. Once soft, add the garlic and cook for another couple of minutes.
Next, stir in the sliced mushrooms and cook for five minutes or until lightly browned and tender. There will be a fair amount of water in this amount of mushrooms, so I'd suggest turning up the heat to cook it all off.
Meanwhile, whisk together the veg stock and flour in a jug or bowl. Combine well to make sure there are no lumps.
Once the mushrooms are tender and most of the water has cooked off, pour in the white wine and reduce for a couple of minutes (still on high heat).
After a couple of minutes, stir in the stock/flour mix, cider vinegar, tamari/soy sauce, mustard, nutritional yeast, bay leaves, salt, and pepper. Stir well, bring to a simmer, and reduce the heat if you previously increased it.
Once the sauce has reached a gentle simmer, add in the coconut milk and continue to simmer over a low heat for 8-10 minutes. Once cooked, the sauce should be thick and creamy. If it seems too thick, add a little water and cook for another couple of minutes.
Adjust the seasoning and serve with your choice of side and chopped fresh parsley. I like it with boiled rice, but you could also have it with pasta (long, wide varieties such as tagliatelle or pappardelle) or mashed potatoes.