Start by wilting the spinach in a large skillet over medium heat, stirring occasionally until completely wilted (about 3-4 minutes). Transfer to a fine-mesh strainer or colander and squeeze very firmly with your hands or the back of a spoon to remove as much liquid as possible—this is crucial for preventing a watery filling. Chop the squeezed spinach finely and set aside. While the spinach cools slightly, mince the garlic and onion as finely as possible, chop the fresh parsley, and dice the chicken into small, uniform 1/4-inch pieces. Having everything prepped before you start cooking ensures the filling comes together quickly.

Preheat your oven to 450°F and line a large baking sheet with parchment paper. Clean the portobello mushroom caps with a damp cloth and use a small spoon to gently scrape out and remove the dark gills from the underside—this prevents the mushrooms from releasing too much moisture during cooking. Remove the stems as well. Place the mushroom caps gill-side up on the prepared baking sheet, then roast for 10 minutes. After 10 minutes, carefully flip them over (gill-side down) and roast for another 5 minutes. This initial roasting helps render some of the mushroom's moisture and sets the caps.

Remove the mushrooms from the oven and carefully pour out any accumulated liquid from the pan. In a medium bowl, combine the parmesan cheese, chopped parsley, minced onion, lightly beaten egg, minced garlic, onion powder, salt, pepper, dried oregano, and red pepper flakes. Mix these dry and aromatic ingredients together thoroughly—this distributes all the seasonings evenly. Gently fold in the cooked spinach and diced chicken until just combined, being careful not to overwork the mixture. I like to use my hands for this final mixing so I can feel when everything is evenly distributed without compacting the filling too much.

Lower the oven temperature to 425°F. Arrange the roasted mushroom caps gill-side down on the baking sheet (this orientation keeps them stable and allows the filling to stay in place). Season the tops of the mushrooms lightly with the remaining 1/4 teaspoon salt and 1/4 teaspoon black pepper. Divide the filling mixture evenly among the six mushroom caps, mounding it slightly on top of each one. Flatten the tops gently with the back of a spoon so the filling sits securely, then lightly spray the filling with cooking spray to help it brown during baking.

Bake the stuffed mushrooms at 425°F for 15-20 minutes, until the filling is heated through and the chicken is nearly cooked. Use a meat thermometer to check the internal temperature of the filling—aim for about 160°F at this stage. Remove the mushrooms from the oven, top each one with shredded or sliced mozzarella cheese, then return to the oven for another 3-5 minutes, just until the cheese melts and the filling reaches 165°F internally. I always check the chicken temperature with a thermometer because the filling can look done before it actually is, especially when it's stuffed inside the mushroom.

While the mushrooms are in the final stage of baking, warm the marinara sauce in a small saucepan over medium heat, stirring occasionally to ensure even heating (about 5 minutes). Once the mushrooms are fully cooked and the cheese is melted, transfer them carefully to a serving platter or individual plates. Pour the warm marinara sauce evenly over each stuffed mushroom, then top with an extra light sprinkle of shredded mozzarella if desired, and garnish with the fresh parsley. Serve immediately while everything is hot.