01 - Preheat the oven to 375°F. Lightly grease a baking dish large enough to hold the peppers upright.
02 - Bring a large pot of salted water to a boil. Add the prepared bell peppers and blanch for 3 minutes. Drain and set aside to cool.
03 - Heat olive oil in a skillet over medium heat. Add the diced onion and sauté for 3 to 4 minutes until softened. Add the minced garlic and cook for an additional 30 seconds until fragrant.
04 - Add the chopped spinach to the skillet and sauté until wilted, about 2 minutes. Remove from heat and let the mixture cool slightly.
05 - In a large mixing bowl, combine the cooked chicken, sautéed onion, garlic, and spinach mixture. Add the ricotta cheese, Parmesan cheese, half of the mozzarella cheese, cream cheese, dried Italian herbs, salt, black pepper, and red pepper flakes if using. Mix thoroughly until all ingredients are evenly incorporated.
06 - Divide the chicken Florentine filling evenly among the blanched bell peppers, packing it generously. Arrange the stuffed peppers upright in the prepared baking dish.
07 - Sprinkle the remaining mozzarella cheese over the top of each stuffed pepper.
08 - Bake for 30 to 35 minutes, or until the peppers are tender and the cheese on top is golden and bubbly.
09 - Remove the baking dish from the oven and let the peppers rest for 5 minutes before serving.