Preheat oven and prepare baking sheet: Preheat oven to 400 degrees. Optionally you can line a baking sheet with aluminum foil for easier clean up.
Squeeze moisture from spinach: Place spinach on several layer of paper towels, spread it out a little then fold paper towels over and squeeze most of the excess moisture from spinach. Place in a medium mixing bowl.
Mix cheeses and seasoning: Add in cream cheese, 1/2 cup parmesan cheese, mozzarella cheese, garlic and season with salt and pepper to taste. Stir mixture to blend.
Oil mushrooms: Place mushroom caps on a baking sheet and spray top and bottom sides with non-stick cooking spray.
Fill mushrooms: Scoop out a spoonful at a time and stuff into mushrooms. Set on baking sheet, filling upright.
Prepare topping, add to mushrooms: In a small mixing bowl stir together panko, remaining 1/4 cup parmesan cheese and 1 1/2 tsp olive oil. Sprinkle over each mushroom.
Bake: Bake in preheated oven until filling is hot, mushrooms have softened and topping is golden brown, about 20 - 24 minutes (if topping didn't brown enough you can broil just briefly). Serve warm.