In a pan with a bit of olive oil, brown your mushrooms. This will take a long time and you will need to do it in batches. I recommend having 2 pans on.
In a separate pot, melt a knob of butter. Add in the leeks, garlic and shallots, and saute until softened. Followed by the browned mushrooms and miso paste. Saute well so that the miso is well incorporated into the mixture.
Deglaze with the white wine and allow the wine to reduce by half, then add in the paprika and thyme. Stir well.
Pour in the stock and cream, cover and let it boil for 5-10 minutes. Season to taste.
Meanwhile, make your cornstarch slurry but mixing cornstarch and water in a 1:2 ratio. The amount of slurry you need will depend on how thick you want to soup to be.
Add the slurry to the pot, a little at a time until the desired consistency is achieved.
Garnish with parsley, heavy cream and a drizzle of truffle oil (optional). Serve with toasted bread slices.