Cut the tofu into 2.5 cm thick rectangles, lay on kitchen paper and drain for 10-15 minutes (turn halfway and change the paper).
In a small saucepan, bring the dashi, soy sauce and mirin to the boil, then turn off the heat.
Sprinkle starch on a plate, lay the tofu on it and sprinkle more on top.
Heat the oil to 180 °C, shake off the excess starch and slide the tofu in one piece at a time; not too many at once or the oil cools. Fry until golden, turning once.
Drain on kitchen paper.
Arrange in a bowl, pour over the sauce and top with grated radish, ginger and spring onion.