Reheat the cooked rice in a steamer or microwave. Put the hot rice in a bowl and toss with the Sushi Vinegar. Let cool.
Transfer the rice to a medium saucepan and pour the Dashi over it; stir to combine. Bring to a boil, uncovered. Reduce the heat to medium-low and simmer for 3 to 4 minutes.
Add the light-colored soy, then pour the eggs right over the rice without stirring. Cook for 10 to 15 seconds, until the eggs are barely set. Remove the pan from the heat.
Divide the rice among individual bowls and top each with wasabi, nori, and mitsuba. Serve immediately.
In a large bowl add all the ingredients and stir to dissolve the sugar. Pour into a covered container. The sushi vinegar will keep for at least 2 months
The night before using the dashi, wipe the kombu clean with a wet kitchen towel to remove any grit, but do not rub off the white powder; much of the flavor lies in its natural MSG. Place in a medium saucepan and add the spring water. Let stand at room temperature overnight.
The next day, remove the kombu and bring the water to a simmer over medium heat. When it reaches a boil, remove from the heat and add the bonito flakes. Let stand until the bonito flakes sink to the bottom of the saucepan, about 15 minutes.
Line a wire sieve with moistened, squeezed-dry cheesecloth and place over a bowl. Strain the dashi through the sieve. Use the dashi within a couple of hours of making. Do not refrigerate or freeze.
* Chef's Note: In this recipe, frozen wasabi paste or paste made from the powder may be substituted if fresh is unavailable.