Step 1: Prepare the Portobello Caps
Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper. Carefully remove the stems from the portobello mushrooms (you can chop and sauté them for the filling if desired). Using a spoon, gently scrape out the brown gills from the underside of each cap. This step is optional but prevents the filling from turning dark and can reduce moisture. Place the caps, gill-side up, on the baking sheet. Brush both sides lightly with olive oil. Season the insides with salt, pepper, and a tiny drizzle of balsamic or soy sauce if using.

Step 2: Par-Bake the Mushrooms
Bake the portobello caps for 10-12 minutes. This step draws out excess moisture, preventing a soggy final dish and concentrates their flavor. Remove from the oven and carefully pour off any liquid that has accumulated in the caps. Keep the oven on.

Step 3: Prepare the Filling
While the mushrooms bake, heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the minced garlic and cook for 30 seconds until fragrant. Add the chopped spinach and cook, stirring, for 2-3 minutes until completely wilted. Transfer the spinach and garlic to a medium mixing bowl.
To the same bowl, add the shredded chicken, softened cream cheese (or ricotta), ¼ cup of the Parmesan cheese, Greek yogurt, Italian seasoning, red pepper flakes, and a pinch of salt and pepper. Mix thoroughly until all ingredients are well combined. Taste and adjust seasoning.

Step 4: Stuff the Mushrooms
Divide the chicken and spinach filling evenly among the par-baked portobello caps, packing it down gently and mounding it slightly.

Step 5: Add the Cheese Topping
In a small bowl, mix the shredded mozzarella with the remaining 2 tablespoons of Parmesan. Sprinkle this cheese blend evenly over the top of each stuffed mushroom.

Step 6: Bake Until Golden and Bubbly
Return the baking sheet to the oven. Bake for 15-18 minutes, or until the cheese is melted, bubbly, and starting to turn golden brown, and the filling is heated through.

Step 7: Broil (Optional)
For an extra-golden top, switch the oven to broil for the final 1-2 minutes, watching closely to prevent burning.

Step 8: Garnish and Serve
Remove from the oven and let cool for 5 minutes. Garnish with fresh chopped parsley or thyme. Serve hot, one stuffed portobello per person as a hearty main course