01 - Preheat oven to 375°F. Lightly grease a baking dish large enough to hold four peppers upright.
02 - Bring a large pot of salted water to a rolling boil. Add the prepared bell peppers and blanch for 3 minutes until slightly softened. Drain carefully and set aside to cool.
03 - Heat 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 and sauté until completely wilted, about 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 and garlic, wilted spinach, ricotta cheese, Parmesan cheese, half of the mozzarella cheese, softened 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, pressing gently to pack the mixture. Arrange the stuffed peppers upright in the prepared baking dish.
07 - Sprinkle the remaining mozzarella cheese evenly over the top of each stuffed pepper.
08 - Bake on the center rack for 30 to 35 minutes, or until the peppers are fork-tender and the cheese topping is melted, golden, and bubbly.
09 - Remove from the oven and let the peppers rest for 5 minutes before serving to allow the filling to set.