Preheat the oven to 375°F.

In a large skillet, heat olive oil over medium heat. Add the sliced onion and green bell pepper, cooking until softened, about 5-7 minutes. Stir in the garlic powder, salt, and pepper.

Add the ribeye steak to the skillet and cook until browned, about 3-4 minutes. Stir in the Worcestershire sauce and remove from heat.

In a mixing bowl, combine the softened cream cheese, provolone cheese, and mozzarella cheese. Mix until well combined.

Spoon the steak and vegetable mixture into each portobello mushroom cap, then top with the cheese mixture. Place the filled mushroom caps on a baking sheet lined with parchment paper.

Bake in the preheated oven for 15-20 minutes, or until the cheese is melted and bubbly.

Remove from the oven and let cool for a few minutes. Garnish with fresh parsley if desired before serving.