1. Cut tops off peppers and remove seeds and ribs. In a mixing bowl, use your hands to gently but thoroughly combine ground chicken, bread crumbs, onion, garlic, egg, oregano, marjoram, salt, and pepper. Divide stuffing in 4 parts and fill pepper cavities, mounding and rounding tops.
2. In a non-reactive saucepan just large enough to hold peppers, bring marinara sauce to a simmer. Add peppers, cover pot, and simmer over medium-low heat until peppers are tender and filling is white throughout, about 45 minutes.