Preheat the oven to 375°F and line a baking sheet with foil or parchment paper.
Clean the mushrooms by gently wiping the caps with a slightly moistened piece of paper towel. Remove the stems and set them aside. Place the mushroom tops on the baking sheet and set them aside.
Mince the mushroom stems and toss with the garlic.
In a large pan or skillet over medium-high heat, melt the butter and add the minced mushrooms and garlic. Sauté for 2-3 minutes and then add the spinach.
When the spinach has wilted, add the cream cheese, basil, salt, pepper, and red pepper flakes. Stir to combine and cook until the cream cheese has melted. Fold in the 1/4 cup of freshly grated parmesan cheese and remove from the heat.
In a small bowl, stir together 2 tablespoons of panko breadcrumbs with 2 tablespoons of grated parmesan cheese.
Gently spoon the spinach mixture into each mushroom cap and then sprinkle with the panko parmesan mixture.
Bake for about 15 minutes or until the mushrooms are tender and the filling is hot and melted.
Garnish with chopped chives or parsley, serve hot, and enjoy!