Heat oven to 300° F
In a medium sized bowl, combine egg, sugar, mirin and soy sauce (or tamari). With a whisk, beat mixture until well blended.
Generously spray baking pan (I use a 9 x 5 silicone loaf pan) with non-stick spray.
Place a strainer above baking pan, and pour egg slowly into strainer. Discard bits of egg that remain.
Bake at 300° F for about 15-20 minutes, or until egg is set.
Remove from oven and allow to cool a bit.
Flip onto a cutting board and trim the sides so that a nice rectangle remains.
Slice egg into any size you wish. Long, thin slices are perfect for sushi rolls.