Rub clean the mushrooms with a damp cloth and trim off any tough ends (particularly from the shitake). Slice the mushrooms and cut slices into small chunks if large. Dice the onion and cut the leek in half then slice.
Warm roughly 1tbsp (14g, ⅓) of the butter in a medium-large pot (around 3.5 - 4 quart/3.2 - 3.8 litres) over a medium heat and add the onion. Cook, stirring now and then, for a minute to start it softening then add the leek. Cook another 3 - 4 minutes until the onion is fairly opaque and softened.
Add another 1 tablespoon of butter then the chopped mushrooms to the pot along with the thyme and some salt and pepper. Carefully stir, let the mushrooms cook down, stir again to bring some of the bottom layer up and get uncooked mushrooms down then cover.
Let the mushrooms cook down for around 3 - 5 minutes, stirring now and then, until they have all become a darker color, are limp-looking and reduced in volume. You may need to add the additional butter during this time to help ensure they don't become too dry.
Add the flour to the mushrooms and stir through to help absorb excess fat (also helps to thicken). Add the brandy and stir to cook off the alcohol.
Pour in the stock, turn up the heat to bring to a simmer then cover and reduce the heat to keep it simmering rather than boiling heavily. Allow the soup to cook for around 8 - 10 minutes until all of the vegetables are cooked through. Remove the thyme stems and remove the pot from the heat.
Either use an immersion blender in the pot or carefully transfer the soup to a blender and blend until smooth. Add the cream and stir through. Check the soup for seasoning and add a little more salt or pepper as needed. If the soup has cooled too much, warm through a little more but try not to boil after adding the cream then serve.