How to Make It
In a large mixing bowl or container, combine all ingredients and blend thoroughly, either using an immersion blender or whisking well by hand. Try to avoid incorporating too much air into the mixture, as it will lead to unattractive bubbles in the tamagoyaki.
Chill the mixture overnight in the refrigerator to allow flavors to meld.
The next day, preheat the oven to 300°F and remove the egg mixture from the refrigerator. Line a 9 x 9-inch baking sheet with walls at least ¼ inch high with parchment paper. Gently re-whisk the eggs for a few seconds to re-incorporate all ingredients, then pour the eggs through a strainer into the baking sheet.
Bake at 300°F for 10 minutes, then lower the temperature to 200°F and continue baking for 1 hour and 15 minutes. The tamagoyaki is done when it’s set along the edges and still jiggling ever-so-slightly in the center, and the top has become an even golden brown color.
Allow tamagoyaki to cool at room temperature, then cut into square or rectangular slices using the tip of a very sharp knife. The remaining tamagoyaki can be sealed in plastic wrap in the refrigerator for 2-3 days.