Soak raw cashews in 2 cups of hot water for 20 minutes. Line a baking sheet with parchment paper. Preheat oven to 350 degrees F.
Wash mushrooms. Remove and dice stems.
Add diced mushrooms stems, onion and minced garlic to a medium skillet over medium heat. Saute for 2 minutes, stirring frequently. Add rosemary and red pepper flakes and stir to combine. Remove from heat. Stir in spinach and set aside.
Drain raw cashews, reserving water. Add cashews, 1/2 cup of reserved water, turmeric, salt and pepper to food processor and process until smooth, stopping to scrape down sides as needed. If needed, add additional reserved water, 1 T at a time until smooth consistency is reached.
In a mixing bowl, combine cooked veggie mixture and cashew puree. Stir until evenly combined.
Evenly fill mushroom caps with mixture and place, filling side up, up parchment-lined baking sheet.