Preheat oven to 400°F (200°C) and line a rimmed baking sheet with parchment paper.
Wipe cremini mushrooms clean with a damp paper towel. Gently snap off stems. Brush outside of mushroom caps lightly with 1 tablespoon olive oil and arrange hollow-side up on the baking sheet.
Heat remaining 1 tablespoon olive oil in a skillet over medium heat. Add minced garlic and cook for 30 seconds until fragrant. Add baby spinach and cook for 1-2 minutes until wilted. Transfer to a clean towel and squeeze out all excess liquid firmly.
In a mixing bowl, combine softened cream cheese, sour cream, squeezed spinach, chopped artichoke hearts, 1/2 cup mozzarella, 1/4 cup parmesan, kosher salt, black pepper, and garlic powder until smooth.
Spoon filling generously into each mushroom cap, mounding slightly on top.
In a small dish, toss panko breadcrumbs with melted butter, remaining 1/2 cup mozzarella, and remaining 1/4 cup parmesan. Sprinkle over tops of stuffed mushrooms.
Bake for 18-20 minutes until mushrooms are tender, filling is hot, and tops are deeply golden brown and bubbling.
Garnish with freshly chopped parsley and serve warm!