Preheat oven to 375°F and line a baking sheet with foil or parchment paper.

Clean mushrooms by dusting off the caps with a damp paper towel. Remove the stems and set aside for chopping. Place mushroom tops on the baking sheet.

Clean mushroom stems by rinsing with water and finely mince them to yield about 2 cups chopped stems.

Measure spinach by packing leaves into the measuring cup, then chop into small pieces.

Peel and mince garlic.

Melt butter in a pan over medium-high heat and add minced mushroom stems and garlic. Sauté for 2-3 minutes then add spinach.

Once spinach has wilted, add cream cheese, Italian seasoning, salt, pepper, and red pepper flakes. Stir until cream cheese is melted.

Remove from heat and fold in the cheese. Adjust spices as needed.

Spoon spinach mixture into each mushroom cap.

Sprinkle with panko breadcrumbs if desired.

Bake at 375°F for approximately 15-20 minutes or until filling is hot and melty and the mushrooms are tender.