01 - Preheat the oven to 375°F. Slice the tops off the bell peppers and remove the seeds and membranes. Lightly brush the outsides with olive oil and set them aside.
02 - Heat 1 tablespoon of olive oil in a large skillet over medium heat. Sauté the diced onion for 2–3 minutes until softened. Add the minced garlic and cook for an additional minute until fragrant.
03 - Add the chopped spinach to the skillet. Cook for approximately 2 minutes, stirring frequently, until the spinach is fully wilted. Remove the skillet from the heat.
04 - In a large mixing bowl, combine the cooked chicken, wilted spinach mixture, ricotta cheese, 1/2 cup of mozzarella cheese, Parmesan cheese, dried Italian herbs, salt, and pepper. Mix thoroughly until all ingredients are evenly incorporated.
05 - Fill each prepared bell pepper evenly with the chicken and cheese mixture. Pack the filling gently into the cavities.
06 - Stand the stuffed peppers upright in a baking dish. Spoon marinara sauce over the top of each pepper and sprinkle with the remaining 1/4 cup of mozzarella cheese. Pour about 1/4 cup of water into the bottom of the dish to help steam the peppers.
07 - Cover the baking dish loosely with aluminum foil. Bake in the preheated oven for 25 minutes.
08 - Remove the foil and return the dish to the oven. Bake for another 10 minutes, or until the cheese is golden and bubbly and the peppers are tender.
09 - Remove the peppers from the oven and allow them to cool slightly for a few minutes before serving.