In a large bowl mix the ground turkey, 1/4 cup of the marinara sauce, peppers, 1/4 cup of the cheese blend, bread crumbs, onion, diced pepperoni (reserve the sliced), egg, garlic and salt until blended.
Pat the mixture over the bottom of a 3 1.2 qt slow cooker (if using a larger cooker, pat the mixture to a 8 inch round).
Spread the top with 1/4 cup marinara sauce.
Cover and cook on low for 4 to 6 hours.
Spread the remaining marinara sauce over the top, sprinkle with the remaining cheese, then top with remaining pepperoni slices.
Cover and cook for 5 minutes or until the cheese has melted.