Preheat your oven to 375°F (190°C) and prepare a baking sheet lined with aluminum foil or parchment paper.

Clean the button mushrooms with a damp cloth, twist off the stems, and set aside to chop later.

In a skillet over medium heat, melt unsalted butter (or olive oil) and add chopped mushroom stems and minced garlic. Sauté for 3-5 minutes until fragrant.

Stir in fresh baby spinach and cook until wilted, about 2-3 minutes. Mix in cream cheese, Italian seasoning, salt, black pepper, and crushed red pepper flakes until well combined.

Fold in the sharp white cheddar cheese, cooking for an additional minute until melted and blended with other ingredients.

Generously stuff each mushroom cap with the creamy filling, leaving a little room for the panko topping.

Sprinkle panko breadcrumbs over each mushroom to add a crispy topping. Lightly toasting breadcrumbs beforehand is optional.

Bake for 15-20 minutes until the caps are tender and the panko topping is golden brown and crispy.

Let cool for a minute, then drizzle with balsamic glaze if desired and serve warm.