01 - Preheat oven to 375°F. Lightly grease a baking dish large enough to hold peppers upright.
02 - Boil a large pot of salted water. Add peppers and blanch for 3 minutes. Drain and let cool.
03 - Heat olive oil in a skillet over medium heat. Sauté onion for 3-4 minutes until softened. Add garlic and cook for 30 seconds.
04 - Add spinach to the skillet and sauté until wilted, about 2 minutes. Remove from heat and let cool slightly.
05 - Combine chicken, sautéed vegetables, ricotta, Parmesan, 1/2 cup mozzarella, cream cheese, Italian herbs, salt, pepper, and red pepper flakes in a bowl.
06 - Fill the peppers evenly with the mixture. Place upright in the prepared baking dish.
07 - Sprinkle remaining mozzarella cheese over the tops.
08 - Bake for 30-35 minutes until peppers are tender and cheese is golden and bubbly.
09 - Let cool for 5 minutes before serving.