Heat 1 tablespoon olive oil in a non stick skillet. Add red pepper, onion, mushroom stems, garlic and Greek seasoning and saute 5-8 minutes until soft. Add chopped spinach to pan and cook until spinach is wilted and cooked down. Set aside to cool. In a medium bowl, combine sauteed vegetables with panko, feta and parmesan.
Preheat oven to 375 degrees. Line up mushroom caps in a baking sheet sprayed with non stick spray. Season mushrooms lightly with salt and pepper. Carefully stuff each mushroom cap with stuffing. Place in oven and cook for approximately 20-25 minutes until mushrooms are browned.
Note: depending on size of mushroom caps, you may have leftover stuffing. Use your leftover stuffing on larger portabella mushroom caps for a quick weeknight meatless meal.