Preheat oven to 350°. In a large bowl, combine the egg, mushrooms, oats, onion, flax and pepper. Crumble turkey and beef over mixture; mix lightly but thoroughly.
Shape into a 10x4-in. loaf. Place in a 13x9-in. baking dish coated with cooking spray. Bake, uncovered, for 50 minutes; drain. Top with marinara sauce. Bake until no pink remains and a thermometer reads 160°, 10-15 minutes longer . If desired, top with Parmesan cheese.