Preheat the oven to 375°F (190°C).

In a skillet, heat olive oil over medium heat. Add minced garlic and sauté until fragrant.

Add chopped spinach to the skillet and cook until wilted. Remove from heat.

In a mixing bowl, combine ricotta cheese, mozzarella cheese, Parmesan cheese, cooked spinach, salt, and pepper. Mix well.

Place the portobello mushrooms on a baking dish, gill side up.

Spoon the cheese and spinach mixture into each mushroom cap, pressing down gently to pack it in.

Bake in the preheated oven for 25 minutes, or until the mushrooms are tender and the cheese is bubbly.

Remove from oven and let cool for a few minutes before serving.