Place a rack in the center of the oven and preheat the oven to 350°F. Place a piece of parchment paper or a silicone mat in a baking sheet and coated with nonstick cooking spray; set aside.
Mix together the spinach, peppers, cream cheese, parmesan cheese, and garlic. Set aside.
Butterfly the chicken then pound to about ¼" thick. Season with salt and pepper and spread them out on a cutting board. Fill each pocket with ¼ of the spinach mixture. Tightly roll up the chicken keeping the cheese mixture inside, then close the seam with toothpicks.
Pour the buttermilk into a shallow bowl. Place the flour in a shallow dish. Place the panko, salt, pepper, and onion powder in a shallow dish. Dip the chicken in the buttermilk then dredge lightly with the flour. Coat the chicken again with the buttermilk. Dredge the chicken in the panko, making sure they adhere well to the surface. Place the chicken on the prepared baking sheet and spray the tops with cooking spray.
Bake 40-45 minutes or until chicken is no longer pink in center and 165°F internal temperature. Let rest for 5 minutes before serving.