Cover the cashews with boiling water and let them soak for 15 minutes. Drain, then blend with 3/4 cup fresh water on high until completely silky, about 1 to 2 minutes. Set the cashew cream aside.
Melt the butter in a large Dutch oven over medium-high heat. Add the mushrooms in an even layer and cook for 8 to 10 minutes, stirring only occasionally, until their liquid has evaporated and they are deeply browned.
Reduce the heat to medium and add the onion. Cook for 5 minutes until soft, then stir in the garlic and thyme and cook for 1 minute until fragrant.
Sprinkle the flour over the mushrooms and stir constantly for 1 to 2 minutes to cook out the raw flour taste. Stir in the soy sauce, then slowly pour in the broth while stirring and scraping up the browned bits from the bottom of the pot.
Bring the soup to a simmer and cook for 15 minutes, stirring occasionally, until slightly thickened.
Transfer about 2 cups of the soup to a blender along with the cashew cream and blend until smooth. Pour it back into the pot and stir well. This keeps the soup creamy while leaving plenty of tender mushroom pieces.
Season with the salt, pepper and lemon juice, then simmer for 2 more minutes. Serve hot topped with fresh parsley.