01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with olive oil or nonstick spray, ensuring it is large enough to hold four peppers upright.
02 - Bring a large pot of salted water to a rolling boil. Carefully lower the prepared bell peppers into the water and blanch for 3 minutes until slightly softened. Remove with tongs, drain well, 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 with the aromatics and sauté, stirring frequently, until completely 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 and garlic, wilted spinach, ricotta cheese, Parmesan cheese, 1/2 cup of the mozzarella cheese, cream cheese, Italian herb blend, 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 four blanched bell peppers, pressing gently to pack the mixture. Arrange the stuffed peppers upright in the prepared baking dish.
07 - Sprinkle the remaining 1/2 cup of shredded mozzarella cheese evenly over the top of each stuffed pepper.
08 - Bake in the preheated oven for 30 to 35 minutes, or until the peppers are fork-tender and the mozzarella topping is melted, golden, and bubbly.
09 - Remove the baking dish from the oven and allow the stuffed peppers to rest for 5 minutes before serving. This helps the filling set and makes for easier plating.