Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper. Clean the portobello mushroom caps with a damp cloth and remove the stems. Gently scrape out the dark gills with a spoon if desired.

Brush both sides of the mushroom caps with 2 tablespoons of olive oil and season with salt and pepper. Place gill-side up on the prepared baking sheet.

Heat remaining 2 tablespoons olive oil in a large skillet over medium heat. Add minced garlic and cook for 30 seconds until fragrant. Add chopped spinach and cook for 2-3 minutes until wilted. Remove from heat.

In a medium bowl, combine the cooked spinach mixture, crumbled feta cheese, breadcrumbs, Parmesan cheese, and red pepper flakes if using. Mix well to combine.

Divide the spinach and feta filling evenly among the four mushroom caps, pressing gently to pack the filling into each cap.

Bake in the preheated oven for 18-20 minutes until the mushrooms are tender and the filling is golden brown on top.

Remove from oven and let cool for 2-3 minutes. Serve warm as a main course with a side salad or as a hearty appetizer.