Place the sweet potatoes in a medium saucepan and add enough water to cover. Bring to a boil, then reduce the heat. Cover and simmer for 10 mins. Add the kohlrabi and leeks.
Cover and simmer about 10 mins more or until the vegetables are just tender. Drain, then transfer to a large bowl and set aside.
Meanwhile, in a small bowl, stir together the apple juice, syrup, cornstarch, cinnamon and ginger.
Transfer the apple juice mixture to the saucepan. Cook and stir over medium heat until the mixture thickens and begins to
gently boil. Cook and stir for 1 more minute.
Add the cooked vegetables to the apple juice mixture. Gently toss until coated, then heat through.