Heat the butter and olive oil in a large pot or Dutch oven over medium heat. Add the onion, carrots, celery, and mushrooms. Cook for about 10 minutes, stirring occasionally, until the vegetables soften.
Stir in the minced garlic, wild rice, poultry seasoning, kosher salt, and white pepper. Cook for 2 to 3 minutes to toast the rice and aromatics.
Sprinkle the flour over the vegetables and stir well. Cook for 2 minutes to remove the raw flour taste.
Pour in the vegetable broth and 2 cups (475 ml) of water. Bring the mixture to a boil, then cover the pot and reduce the heat to low. Simmer for about 90 minutes, or until the wild rice is tender.
While the soup simmers, place the raw cashews in a heat-safe bowl and cover them with hot water. Let them soak for 15 minutes, then drain the water.
Transfer the drained cashews to a blender. Add 1 cup (240 ml) of fresh water and blend on high speed until the mixture is very smooth.
Once the wild rice is tender, stir the cashew cream, slivered almonds, and dry sherry into the soup. Taste the broth and add more salt or pepper if needed. Serve warm.