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 onion and sauté for 3 to 4 minutes until softened. Add the garlic and cook for another 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 with ricotta cheese, Parmesan cheese, 1/2 cup 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 - Fill each blanched bell pepper evenly with the chicken Florentine mixture. Arrange the stuffed peppers upright in the prepared baking dish.
07 - Sprinkle the remaining 1/2 cup of 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 from the oven and let the peppers rest for 5 minutes before serving.