Rinse the Japanese Short-Grain Rice under cold water until the rinse runs clear, then combine with Water in a rice cooker or pot and cook according to your usual method; keep warm.
Prepare the tempura ingredients: peel and slice the Sweet Potato into 1/4" rounds, cut the Kabocha Squash into thin wedges, halve the Japanese Eggplant lengthwise, remove stems from the Shiitake Mushrooms, trim the Green Beans, and devein and pat dry the Shrimp.
Make the batter just before frying: in a bowl gently combine equal parts All-Purpose Flour and Cake Flour, add a lightly beaten Egg and cold water (keep it very cold). Stir a few times with chopsticks or a fork—do not overmix; a few lumps are good for a light, airy batter.
Heat the Neutral Oil in a deep pan to 170–180°C (340–360°F) or medium-high; you can test with a small drop of batter—if it sizzles and rises slowly, the oil is ready.
Dust the prepared shrimp and vegetables lightly with a bit of flour, then dip each piece into the batter and carefully lower into the oil in small batches so the temperature holds. Fry until the batter is pale golden and crisp (shrimp 2–3 minutes, thin veggies 1–3 minutes depending on thickness). Drain on a wire rack or paper towel.
Make the tentsuyu dipping sauce: in a small saucepan warm Dashi with Soy Sauce, Mirin and a little Sugar just until the sugar dissolves. Remove from heat and keep warm or cool to room temperature, as you prefer.
Grate the Daikon and Ginger and serve small mounds beside the dipping sauce for bright, refreshing condiments.
Serve the hot tempura immediately with the steamed rice and tentsuyu on the side—dip each piece into the sauce and add a bit of grated daikon or ginger to taste for the authentic finish.
