Preheat your oven to 400°F (200°C) and line a baking sheet with foil or parchment so cleanup’s easy.
Pull the stems off the mushrooms and scrape out the gills; brush the caps with olive oil and sprinkle with salt and pepper.
Lay the mushrooms on the baking sheet, gill side up.
In a bowl, mix together the chopped spinach, cream cheese, mozzarella, Parmesan, garlic, and oregano until it’s all smooth and creamy.
Spoon the filling into each mushroom cap and press it down a bit so it stays put.
Bake them for about 18–20 minutes, until the tops are golden and the cheese is bubbling.
Let them sit for about 5 minutes so they’re not scorching hot.
Add a little fresh parsley on top if you want, then dig in.