Preheat oven to 180°C and grease or line a baking sheet or oven dish with aluminum foil (I usually lightly grease the foil with baking spray).
Clean the mushrooms: gently wipe the tops with a dry kitchen towel and carefully scrape the gills out. The gills are perfectly edible, but will release quite some moisture during baking (and you can fit more filling into the mushroom... win/win).
Roughly chop the spinach.
Mix spinach, cream cheese, grated cheese, salt and pepper in a bowl.
Divide the filling among the mushrooms.
Bake ca. 20-25 minutes in the middle of the oven.