Heat the oil and butter in a large saucepan before adding the chopped onions and leeks. Cook for 8-10 minutes.
Once the onions and leeks have started to soften, add the chopped garlic and cook for another 2-3 minutes.
Next, add the mushrooms and stir these well for 4-5 minutes or until they begin to soften.
Once soft, add in the bay leaves, tarragon, and vegan stock, and season with salt and pepper. If using wholegrain mustard and nutritional yeast, also add these. Bring to a boil, then reduce the heat and simmer for 10 minutes.
To thicken the soup (optional), first combine the cornflour with equal parts of cold water in a separate jug or bowl. Mix well and then stir this into the soup.
Cook for a couple of minutes, then add the plant-based milk and adjust the seasoning. Turn off the heat, remove the bay leaves, and blitz using a hand blender. You could also transfer to a larger, more powerful blender, but I prefer some texture in this particular soup.
Serve with chopped fresh parsley and crusty bread.