Stir the dashi, mirin, soy sauce, sugar and salt together until the sugar dissolves; let cool if warm. Crack the eggs into a large measuring cup, add the seasoning and whisk gently without whipping in air. Strain through a fine-mesh sieve.

Heat an 8-inch nonstick skillet over medium-low. Wipe the surface with a paper towel dipped in oil. The pan is ready when a few drops of egg sizzle softly and set without browning.

Pour in 3-4 tablespoons of egg, just enough to thinly coat the bottom. Tilt to spread. When the bottom sets and the top is still glossy, about 30 seconds, roll the sheet toward you with a spatula or chopsticks and push the roll to the far side of the pan.

Re-oil the empty part of the pan, pour in another thin layer (lifting the roll so egg flows underneath), and roll it around the existing cylinder when it sets. Repeat for 5-6 layers until all the egg is used.

On the final roll, press gently with the spatula to square the sides. Transfer to a bamboo sushi mat or cutting board, wrap snugly for 2-3 minutes to set the shape.

Cut crosswise into 6-8 pieces about 3/4 inch thick. Serve warm, at room temperature, or chilled.