Preheat the oven to 375°F (190°C).
Clean the portobello mushrooms with a damp paper towel and remove the stems.
Heat 1 tablespoon of olive oil in a skillet over medium heat. Sauté garlic until fragrant.
Add chopped spinach to the skillet, cooking until wilted. Season with salt, pepper, and oregano.
In a mixing bowl, combine ricotta, mozzarella, Parmesan, and the sautéed spinach mixture. Stir until combined.
Drizzle the portobello caps with remaining olive oil and season with salt and pepper.
Stuff each portobello cap with the cheese and spinach mixture, packing it in gently.
Place the stuffed mushrooms on a baking sheet and bake for 25-30 minutes, or until mushrooms are tender.
Remove from the oven and let cool slightly before serving.