Melt the butter in a large saucepan on medium heat. Add the onions and garlic and cook for 4 minutes, or until the onion has started to soften.
Slice the mushrooms and add them to the pan, along with the thyme and tarragon. Cook for 5 minutes or so, until the mushrooms have browned and started to release their liquid and shrink. Give them a stir now and then.
Whisk the flour and milk together in a jug until smooth, then pour into the saucepan. Add the freshly boiled water, vegetable bouillon power and black pepper. Bring to the boil and stir continuously for 1 minute, until the soup starts to thicken. Reduce the heat to low, partially cover with a lid and gently simmer for 10 minutes. Stir occasionally with a wooden spoon, to prevent the soup from sticking to the bottom of the pan.
Remove from the heat and stir in most of the parsley (leave some for a garnish). Taste test and add more freshly ground black pepper, if desired. Leave to cool for a couple of minutes (the soup will thicken as it cools).
Transfer half the soup to a jug blender (or use a bowl and immersion blender) and whizz until smooth, before adding it back into the pan and warming through.
Divide the soup between four warm bowls. Add a sprinkle of chopped parsley for garnish and serve. Any leftovers will keep for three days stored in an airtight container in the fridge.