Combine mushrooms, garlic, 2 ½ tablespoons parsley, salt, pepper, red pepper flakes and olive oil in a medium mixing bowl. Toss to coat. Cover and let sit for 20 minutes.
Drizzle Blackstone with a little oil and butter, spreading around, on medium high heat. Add the mushrooms in single layer and let cook 3 minutes. Flip and cook additional 2 minutes. Plate and garnish with reserved parsley.
