Make the soy-marinated egg
Bring a pot of water to a boil and carefully add the egg.
Cook for 6 minutes for a jammy centre, then immediately transfer to an ice bath.
Once completely cooled, peel the egg and place it in a container.
Add enough light soy sauce, mirin and water to partially cover the egg, along with a little sugar and a dash of dark soy sauce.
Cover and refrigerate for at least 6 hours, preferably overnight.
Prepare the beef
If using a steak, place it in the freezer for around 30–45 minutes until partially frozen. This makes it much easier to slice very thinly.
Slice against the grain into thin pieces.
If using pre-sliced hot pot beef, you're already good to go!
Brown the tofu & spring onions
Heat a little neutral oil in a pan over medium heat.
Add the tofu cubes and spring onion whites and pan-fry until the tofu is golden brown on the outside.
Remove the spring onion whites and set aside. Continue cooking the tofu until nicely browned on all sides, then remove and set aside.
Cook the onions
Using the same pan, add the sliced onion and sauté for a couple of minutes until it starts to soften.
Build the sauce
Add the light soy sauce, mirin, sake, water, dashi powder and sugar.
Bring everything to a gentle boil, stirring to dissolve the sugar and combine the flavours.
Cook the beef & tofu
Add the thinly sliced beef to the simmering sauce and gently stir to coat.
Return the tofu to the pan and allow everything to simmer together for 1–2 minutes, or until the beef is cooked through.
Don't overcook the beef, since they're quite thin, they'll barely need any time at all.
Taste the sauce and adjust if needed.
Assemble the bento
Add a generous layer of fluffy steamed rice to your bento box or lunch container.
Make a little indent in the rice and spoon over the beef, onions and tofu, making sure to get plenty of that gorgeous sauce over the rice. 👀
Top with the pickled ginger, reserved spring onion whites and sliced spring onion greens.
Finally, cut your soy-marinated egg in half and place it on top.
Serve warm or enjoy at room temperature - just like Rengoku would!
