Blend cashews with 1 cup water until creamy. Add 1 to 2 tablespoons more water if needed to blend well.
Preheat the oven to 350 degrees F (175 degrees C). Spray a baking sheet with cooking spray.
Clean mushrooms with a damp paper towel. Carefully break off stems and reserve the caps. Chop stems very finely, discarding tough ends.
Heat oil in a large skillet over medium heat. Add chopped mushroom stems and garlic to the skillet. Fry until any moisture has disappeared, taking care not to burn garlic, about 3 minutes. Set aside to cool.
Stir cashew cream, nutritional yeast, salt, black pepper, onion powder, and cayenne pepper into the cooled mushroom mixture. Use a little spoon to fill each mushroom cap with a generous amount of stuffing. Arrange the mushroom caps on the prepared baking sheet.
Bake in the preheated oven until mushrooms are piping hot and liquid starts to form under the caps, about 20 minutes.