01 - Preheat the oven to 375°F. Lightly grease a 9×13-inch baking dish with olive oil or nonstick spray, ensuring it is large enough to hold the peppers upright.
02 - Bring a large pot of salted water to a rolling boil. Carefully add the prepared bell peppers and blanch for 3 minutes until slightly softened. Drain thoroughly and set aside to cool.
03 - Heat the olive oil in a large skillet over medium heat. Add the diced onion and sauté for 3 to 4 minutes until translucent and softened. Add the minced garlic and cook for an additional 30 seconds until fragrant.
04 - Add the chopped spinach to the skillet with the aromatics and sauté, stirring frequently, until fully wilted, approximately 2 minutes. Remove the skillet from heat and allow the mixture to cool slightly.
05 - In a large mixing bowl, combine the shredded chicken, sautéed onion-garlic-spinach mixture, ricotta cheese, Parmesan cheese, half of the mozzarella (1/2 cup), cream cheese, dried Italian herbs, salt, black pepper, and red pepper flakes if using. Fold everything together until evenly incorporated.
06 - Spoon the chicken Florentine filling evenly into each blanched bell pepper, packing gently. Arrange the stuffed peppers upright in the prepared baking dish.
07 - Sprinkle the remaining 1/2 cup of mozzarella cheese evenly over the top of each stuffed pepper.
08 - Bake uncovered for 30 to 35 minutes, until the peppers are fork-tender and the cheese on top is melted, golden, and bubbling.
09 - Remove the dish from the oven and let the peppers rest for 5 minutes before serving. This allows the filling to set and makes for easier plating.