Preheat the oven to 375 degrees F (190 degrees C).

Lightly grease 30 muffin cups with olive oil. Brush all sides mushrooms with remaining olive oil; place gill-sides-up into the prepared muffin cups.

Bake in the preheated oven until mushrooms just tender, about 12 minutes.

Meanwhile, beat egg, garlic, salt, and black pepper together in a bowl; stir in spinach, 1/4 cup Parmesan cheese, Gouda cheese, mozzarella cheese, and bread crumbs until evenly combined.

Remove mushrooms from oven; drain and discard any accumulated juice. Fill mushroom caps with spinach mixture; return to the muffin cups. Sprinkle mushrooms with remaining 1/4 cup Parmesan cheese.

Bake until stuffing is hot and golden brown, about 10 minutes.