Optional – make mushroom stock: Place the dried mushrooms in a bowl and cover with 1 ½ cups (360 mL) room temperature water. Place a smaller bowl or something heavy on top to ensure they stay submerged. Soak for 30 minutes, or until softened.
Scoop out the mushrooms. Strain the liquid through a fine mesh sieve to catch any sediment, reserving the stock.Add the rehydrated mushrooms to a bowl of water and agitate with your hands. Drain the water and repeat a few times until the sediment is gone. This ensures the grit won’t make it into the soup. Slice mushrooms and set aside. For fresh mushrooms like cremini and shiitake, slice them (remove shiitake stems). For oyster or maitake mushrooms, tear them into strips with your hands, discarding any tough ends.
Heat a Dutch oven or soup pout with the olive oil over medium-high heat. Add the onions with a pinch of salt, and saute for 4 to 5 minutes, or until starting to soften. Add the garlic and cook for another 1 to 2 minutes. Add the butter to the pan, and once melted, add the fresh mushrooms and rehydrated dried mushrooms, if using. Cook for about 5 minutes, until softened. Season with salt and pepper at the end. Pour in the white wine and use it to scrape up any browned bits on the bottom of the pot. Simmer for 3 to 4 minutes, or until the smell of alcohol has dissipated and some of the liquid has bubbled off.
If using, pour in the mushroom stock but reserve the last tablespoon or so, as it’s usually sediment-heavy. Add the vegetable broth too (use 4 cups if not using mushroom stock). Add the bouquet garni and bring to a boil. Cover, reduce the heat, and simmer for 15 minutes.
Scoop out ⅓ to ½ of the soup (about 2 ½ cups or 550 to 600g), including some mushrooms but not the bouquet garni. Transfer this to a blender and add the cashews, miso and soy sauce. Start slowly then move to high speed and blend until smooth.
Stir the blended soup back into the remaining soup in the pot. Bring to a simmer and simmer for 5 to 10 minutes, until the flavors have developed and the soup has thickened slightly. Season with salt and pepper. If you want a slight pop of acidity, add the Dijon mustard. Remove the bouquet garni.
Ladle soup into serving bowls. Top each with a tiny drizzle of good-quality olive oil, if a little extra richness is desired, and garnish with a small handful of chopped parsley/chives.