Chop the sweet potato (2-3mm), peel and slice the lotus root into 2mm halfmoons. Soak separately in water to remove the bitterness.
Heat vegetable oil in a frying pan, and fry the sweet potato and lotus root over medium heat until the moisture has thoroughly evaporated.
Toss the vegetables in oil, then add water, sugar, sake and soy sauce. It's done when the liquid has completely evaporated. Garnish and serve with sesame seeds to taste.