Preheat the grill to medium heat. Grill the franks according to package directions. Grill the buns, cut-side down, until golden brown.
Spread the marinara sauce inside the buns. Add some mozzarella and bell peppers to each bun. Place on the grill, and close the lid. Grill until the cheese melts, about 2 minutes. Remove from the grill. Add the franks to the buns and top each with some pesto.