In bowl combine cream cheese, garlic powder, minced onion, dried oregano, salt, pepper, diced mushroom stems and bell pepper.
Spoon stuffing onto tops of portobello mushrooms. Brush sides of mushroom with some olive oil.
Bake at 400°F for about 10-12 minutes or until filling becomes warm and bubbly. Top with about 1-2 Tablespoons of tuna, feta cheese and bread crumbs on each mushroom. Bake for another 3-5 minutes until toppings are heated through.
Top with parsley. Optional, serve on a bed of arugula.