01 - Preheat the oven to 375°F. Slice the tops off the bell peppers and remove the seeds and membranes. Brush the peppers lightly 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 soft, then add the minced garlic and cook for an additional minute until fragrant.
03 - Add the chopped spinach to the skillet and cook for about 2 minutes until wilted. Remove the skillet from the heat.
04 - In a large bowl, combine the cooked chicken, spinach mixture, ricotta cheese, 1/2 cup of mozzarella cheese, Parmesan cheese, Italian herbs, salt, and pepper. Mix thoroughly until all ingredients are well incorporated.
05 - Fill each prepared bell pepper evenly with the chicken and cheese mixture.
06 - Arrange the stuffed peppers upright in a baking dish. Spoon marinara sauce over the top of each pepper and sprinkle with the remaining mozzarella cheese. Pour 1/4 cup of water into the bottom of the dish to create steam and prevent sticking.
07 - Cover the baking dish loosely with aluminum foil and bake for 25 minutes. Remove the foil and bake for another 10 minutes, or until the cheese is golden and the peppers are tender.
08 - Allow the stuffed peppers to cool for a few minutes before serving.