Pour the remaining 1 cup marinara sauce around the peppers and lightly over the tops, letting it run down into the bottom of the slow cooker. Cover with the lid.
Cook on LOW for 4 to 6 hours or on HIGH for 2 1/2 to 3 1/2 hours, until the peppers are tender and the sausage filling is cooked through and no longer pink in the center.
About 15 minutes before serving, sprinkle the remaining 1/2 cup shredded mozzarella
