Preheat oven to 400 F. In a food processor, add the eggs, hanpen cut into bite-sized pieces, and the seasonings. (If using white fish, process the fish before adding the rest of the ingredients.) Mix until smooth, and then strain.
Cover the oven tray (10x10) with a baking sheet, and pour in the mixture. Bake for 25 minutes on the top rack until brown. Insert a toothpick and if nothing gets stuck to it, it’s done.
Peel the baking sheet off, and place the datemaki brown side down on an oni-sudare makisu bamboo mat. Make shallow cut lines vertically, about 1/2 inches apart, and roll tightly. Tighten the roll with rubber bands, and leave to cool.
Slice to serve.