Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper. Gently clean the mushrooms by wiping them with a damp paper towel. Carefully remove the stems by wiggling them free. Arrange the mushroom caps hollow-side up on the prepared baking sheet.

Finely chop the mushroom stems. Heat olive oil in a medium skillet over medium heat. Add the chopped mushroom stems and finely chopped onion. Sauté for 5-7 minutes, until softened and the moisture has cooked off. Add the minced garlic and cook for another minute until fragrant. Remove from heat and let it cool slightly.

In a large mixing bowl, combine the softened cream cheese, mayonnaise, and sour cream. Mix with a spatula until smooth. Stir in the Parmesan cheese, mozzarella cheese, drained and chopped artichoke hearts, and the thoroughly squeezed spinach.

Add the cooled mushroom stem and onion mixture to the cheese mixture. Season with black pepper, red pepper flakes (if using), and a pinch of salt. Stir everything together until well combined.

Using a small spoon or a 1-tablespoon cookie scoop, generously fill each mushroom cap with the spinach artichoke mixture, mounding it slightly on top.

In a small bowl, toss the panko breadcrumbs with the melted butter until evenly coated. Sprinkle the breadcrumbs and a little extra Parmesan cheese over the top of each stuffed mushroom.

Place the baking sheet in the preheated oven and bake for 20-25 minutes, or until the filling is hot and bubbly and the panko topping is golden brown.

Let the mushrooms cool for 5 minutes on the baking sheet before transferring to a serving platter. Garnish with fresh chopped parsley and serve warm.