Preheat oven to 400°F. Brush portobello caps with 1 tablespoon olive oil, season with salt and pepper, and place gill-side up on a baking sheet.
Heat remaining oil in a skillet over medium heat; cook onion 4 minutes until softened, then add garlic and cook 1 minute.
Add spinach in batches, stirring until wilted, about 3 minutes; let cool slightly.
Stir in feta, ricotta, breadcrumbs, oregano and red pepper flakes.
Divide filling among mushroom caps, pressing gently to pack.
Bake 18-20 minutes until tender and golden on top.
Garnish with parsley before serving.