Preheat oven to 375°F (190°C). Gently wipe portobello mushrooms clean with a damp cloth. Carefully remove the stems and use a spoon to scrape out the dark gills from the underside of the mushroom caps. Brush the mushroom caps with 1 tablespoon of olive oil, both inside and out. Place them gill-side up on a baking sheet.

In a medium bowl, combine the chopped spinach, crumbled feta cheese, chopped sun-dried tomatoes, minced garlic, bread crumbs, and grated Parmesan cheese. Drizzle with the remaining 1 tablespoon of olive oil. Season with salt and black pepper to taste and mix well until all ingredients are evenly distributed.

Evenly divide the spinach and feta mixture among the four portobello mushroom caps. Press the filling gently into each mushroom cap, creating a small mound.

Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the filling is golden brown and heated through. The mushrooms should release some liquid, which is normal.

Carefully remove the stuffed portobello mushrooms from the oven. Let them cool for a few minutes before serving. They can be served as a main course or a substantial side dish.