Soak the cashews in a bowl of water for at minimum 1 hour or up to 8 hours.
Preheat the oven to 375ºF. Line a rimmed baking sheet with parchment paper.
Heat the butter in a large skillet over medium heat. Add the garlic, shallot, and onion and cook until soft, about 3 minutes. Fold in the spinach and cook, stirring occasionally, until the spinach has wilted. Set aside.
To make the cashew cream: Drain the soaked cashews and place them in a small food processor or high-speed blender. Puree the cashews with the 1 tablespoon of olive oil and ½ teaspoon of salt. Fold the cashew cream into the spinach mixture until evenly combined.
Arrange the mushroom caps on the lined baking sheet and brush the outside of the caps lightly with olive oil. Scoop a large spoonful of the spinach mixture into the cavity of each cap. Top with the cheese, if using.
Bake in the oven for 15 to 20 minutes or until the mushrooms are soft and the cheese has begun to brown. Serve warm or at room temperature.
Variation: Hot Spinach Dip. Egg-free and dairy-free (if cheese is omitted), this is your solution to a hot, creamy party dip that everyone can enjoy. Place dip into an oven-safe bowl, top with cheddar cheese, if desired, and bake until the cheese is golden. Use homemade crackers (page 000), veggies, or plantain chips for dipping.