Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Brush the portobello mushroom caps with olive oil and arrange them on the prepared baking sheet, gill-side up.
Bake the mushrooms for about 10 minutes, allowing them to release excess moisture. Carefully drain away any liquid from the baking sheet.
Meanwhile, heat a skillet over medium heat. Add the garlic and chopped onion and sauté for 3–4 minutes, or until softened.
Add the fresh spinach and cook for 2–3 minutes, stirring occasionally, until wilted. Remove the skillet from the heat.
In a mixing bowl, combine the spinach and onion mixture with the ricotta, Parmesan, salt, pepper, and a small pinch of nutmeg.
Spoon the creamy filling evenly into the mushroom caps.
Sprinkle the shredded mozzarella over the top of each mushroom.
Return the mushrooms to the oven and bake for 15–20 minutes, or until the filling is hot and the mozzarella is bubbly and golden.
Let the mushrooms rest for a few minutes before serving. Enjoy the Spinach and Cheese Stuffed Portobello Mushroom while warm.