Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.

Gently clean the portobello mushroom caps and brush them with olive oil. Arrange them gill-side up on the prepared baking sheet.

Bake the mushrooms for 10 minutes, or until they begin releasing moisture. Carefully drain away any excess liquid.

Meanwhile, heat a skillet over medium heat. Add the garlic and chopped onion and sauté for 3–4 minutes, until softened and fragrant.

Add the fresh spinach and cook for 2–3 minutes, stirring occasionally, until completely wilted. Remove the skillet from the heat.

In a mixing bowl, combine the cooked spinach mixture with ricotta and Parmesan. Season with salt, black pepper, and a small pinch of nutmeg.

Divide the creamy spinach mixture evenly among the mushroom caps.

Sprinkle shredded mozzarella over each stuffed mushroom.

Return the Spinach and Cheese Stuffed Portobello Mushrooms to the oven and bake for 15–20 minutes, until the mushrooms are tender and the mozzarella is melted, bubbly, and lightly golden.

Let them rest for a few minutes before serving warm.