Preheat the oven to 450 degrees Fahrenheit.

Clean the Portobello mushrooms by brushing off any dirt. Snip off the stems and discard the black gills.

Place the mushrooms on a baking sheet and brush both sides with olive oil. Season lightly with salt and pepper. Flip the mushrooms gill side down and bake for 10 minutes.

While the mushrooms are baking, finely chop the reserved mushroom stems. Heat oil in a skillet and cook garlic and onion until softened. Add the mushroom stems and cook for 2-3 minutes.

Add chopped spinach to the pan and cook until water from the spinach has evaporated, about 5 minutes. Let the mixture cool slightly.

In a bowl, combine the cream cheese, ? cup mozzarella, Italian seasoning, paprika, salt, pepper, and the cooked onion-spinach mixture. Mix well.

Flip the baked mushrooms cavity side up and fill with the spinach mixture. Top each with the remaining mozzarella cheese.

Bake for an additional 10-12 minutes until the stuffing is bubbly and the cheese has melted fully.

Serve hot with a sprinkle of fresh parsley on top.