Preheat the Oven: Preheat the oven to 375°F (190°C).
Prepare the Mushrooms: Clean the portobello mushrooms with a damp paper towel and remove the stems and gills. Drizzle the mushroom caps with olive oil and season with salt and pepper. Place them on a baking sheet and roast in the preheated oven for 10 minutes, until they begin to soften.
Sauté the Spinach: In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the garlic and sauté until fragrant, about 1 minute. Add the fresh spinach and cook until wilted, about 3-4 minutes. Season with salt, pepper, and a pinch of nutmeg. If using frozen spinach, cook until heated through.
Make the Filling: In a large bowl, combine the sautéed spinach with the goat cheese and chopped sun-dried tomatoes. Mix until the goat cheese is evenly distributed and the filling is creamy.
Stuff the Mushrooms: Remove the mushrooms from the oven and carefully spoon the spinach and goat cheese mixture into each mushroom cap. Top with a few extra pieces of sun-dried tomato for garnish.
Bake: Return the stuffed mushrooms to the oven and bake for an additional 10-12 minutes, until the filling is heated through and the tops are slightly golden.
Serve and Garnish: Remove the stuffed mushrooms from the oven and let them cool slightly before serving. Garnish with fresh parsley or basil and a drizzle of olive oil.