Preheat oven to 350. Position a rack in the middle of the oven. In a mixing bowl, combine 2/3 cup mustard, pickle relish, bread crumbs, eggs, onions, salt, and pepper. Mix well. Add chicken and mix until combined. Divide mixture between two 8 "x 4" loaf pans, mounding it slightly. Bake loaves for one hour, or until loaves reach 185 on a meat thermometer. In a small bowl, whisk together the remaining mustard and brown sugar. Brush the mixture evenly over the top of the meat loaves and bake until the topping is glazed, about five minutes. Let the loaves rest on a rack for ten minutes before slicing.