Cut the tops off the bell pepper and carefully remove the seeds from the center to create a bowl.
In a large bowl combine the ground turkey, garlic powder, Italian seasoning, black pepper, egg, and Panko. Mix well and stuff the mixture inside each of the bell pepper bowls.
Spray a slow cooker with non-stick spray and place each stuffed pepper inside. Pour the marinara over the top. Cook for 4 hours on high or 6 hours on low. Serve hot.