1. Mix the first 7 ingredients in a medium bowl. Add the chilled water and whisk briefly to just blend the ingredients. Refrigerate for 30 minutes before frying.
2. Steam the sweet potato or potato and set aside to cool. In a small saucepan bring the sauce ingredients to a boil, whisking constantly. Cook for a minute, then remove from the heat and set aside to cool.
3. To fry the tempura, heat the oil in a medium saucepan or wok to frying temperature (365-375°F) on medium-high heat. Evenly coat 8 pieces of vegetable with batter, carefully place in the hot oil, and fry until golden brown. Drain well on paper towels. Repeat until finished.
4. To serve, arrange the tempura on a platter and smother liberally with tempura sauce, or use the sauce as a dip on the side. Tempura is best eaten right away while crisp and hot.
*To make fresh ginger juice, simply grate some fresh ginger, then manually squeeze the pulp.
Note: For an Indian variation, add 1/2 tsp. ground cumin, 1/4 tsp. ground coriander, and 1/4 tsp. red chili flakes to the batter.