Preheat your oven to 425°F (220°C) and grease a rimmed baking sheet with olive oil or cooking spray.

Remove the stems and gills from the portobello mushrooms. Brush each mushroom cap with olive oil and place them cap-side down on the baking sheet.

In a skillet, sauté diced yellow onion, minced garlic, chopped mushroom stems, and diced red bell pepper in olive oil until tender.

In a bowl, combine cream cheese and sour cream, then fold in the sautéed mixture, cooked chicken, chopped spinach, half of the mozzarella, and Parmesan. Season with salt and pepper.

Fill each mushroom cap with the creamy filling and sprinkle the remaining mozzarella on top.

Bake the filled mushrooms for 12-16 minutes until the cheese is golden and bubbly.