Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Clean mushrooms and remove stems. Scrape out gills with a spoon if desired.
Brush mushroom caps with 1 tablespoon olive oil and season lightly with salt and pepper.
Place mushrooms gill-side up on prepared baking sheet.
Heat remaining olive oil in a large skillet over medium heat.
Add diced onion and cook for 3-4 minutes until softened.
Add garlic and cook for 1 minute until fragrant.
Add chopped spinach and cook until wilted, about 2-3 minutes.
Remove from heat and let cool slightly.
Stir in crumbled feta, panko breadcrumbs, parsley, oregano, salt, pepper, and pine nuts.
Divide filling evenly among mushroom caps, mounding slightly.
Sprinkle with Parmesan cheese.
Bake for 15-18 minutes until mushrooms are tender and filling is golden.
Let cool for 5 minutes before serving.
