Remove giblets from hens and discard. Rinse hens under cold water and pat dry. Using a long, sharp knife, split the hens lengthwise. You may also buy precut hens. Place the hens, cavity side up, in a roasting pan. Dilute 1/4 cup of the apple juice concentrate with the water. Pour juice over the hens. Bake uncovered at 350 degrees F for 45 minutes. Turn breast side up.
In a small pan over medium heat, combine the remaining apple juice concentrate, cornstarch and cinnamon; mix well. Add 2 lemon slices and continue cooking until thickened. Remove from heat and brush hens with the sauce. Return the hens to the oven and continue to bake for an additional 15 minutes.
Transfer the hens to a serving platter and garnish with remaining lemon slices.