Preheat your oven to 350°F (180°C) and line a baking tray with a parchment sheet. *3
Mixing with a Blender
Cut the hanpen into little cubes and add it to the mixer.
Put all the ingredients into a blender or a food processor.
Mix all the ingredients together for about 30 seconds to 1 minute.
Leave the mixture to settle until the bubbles go down.
Pour the mixture into the baking tray.
Mixing with a Whisk manually
Squish the hanpen over the package.
Empty the package of the hanpen into a large mixing bowl.
Add one egg at a time and whisk well each time adding the eggs.
Add all seasonings and mix well with the whisk.
Pour the mixture into the prepared baking tray.
Baking *4
Bake in the preheated oven for 20 minutes.
Turn the oven off and take out the omelette. While it is still hot, place the rolling mat (flat side face up) over the omelette and quickly and carefully flip it over one hand on the omelette and one hand on the mat.*5
Remove the baking paper and score the omelette from the edge about 0.4"-0.6"(1-1.5cm) apart in order to roll it nicely without breaking.
Roll it into a cylinder shape with the rolling mat and secure it with a rubber band on each end to hold the shape.
Look from the side and place the egg roll closed side down, or leave it standing vertically. Let it cool slightly then wrap it with cling wrap and refrigerate the rolled omelette overnight, then remove from the sushi mat and slice into 0.8inch (2cm) thick and serve!
Notes
*1 One hanpen that I used weighed 2.8oz (80g), and I used two of them.*2 If you can't access any of the ingredients, see this Japanese food substitute post. *3 My baking mold size 12x12 inch (30x30cm)*4 You can use a frying pan too. Cook it over low heat for about 15min with an aluminium foil cover, flip it over carefully and cook the other side if necessary. *5 Rolling into the shape needs to be done while the omelette is still hot otherwise it will break when it is rolled. Just be careful not to burn yourself.* 6 Dashi - look here*7 Cooking time does not include setting time. *8 Nutrition information is for a whole rolled omelette.