Heat a frying pan over medium-high heat until hot. Add the 1 teaspoon vegetable oil and then add the 175 grams tofu and 100 grams Tokyo Negi in a single layer. Fry these undisturbed until they're browned on one side (~2 minutes). Flip them over and then push them to one side of the pan.
1 teaspoon vegetable oil, 175 grams tofu, 100 grams Tokyo Negi
Add the 100 grams onions along with the ¼ cup sake, ¼ cup mirin, ¼ cup soy sauce, and 2 tablespoons evaporated cane sugar. Let these simmer over medium heat until the onions are tender (~6 minutes). Flip the ingredients over periodically to ensure they get seasoned evenly.
100 grams onions, ¼ cup sake, ¼ cup mirin, ¼ cup soy sauce, 2 tablespoons evaporated cane sugar
Add the 250 grams beef and stir it around in the braising liquid to cook it through. The Gyunabe is done when the meat is no longer pink. Don't overcook it, or the beef will get tough.
250 grams beef
To pack the bento, add a layer of the 2 servings cooked short-grain rice to the bottom of each bento box and top the right ⅔ with the Gyunabe.
2 servings cooked short-grain rice
Add some Benishoga to the top left edge of the box and then stagger some tofu on top of it.
Benishoga
Layer some of the Negi on top of the tofu, and then finish your Demon Slayer bento box off with half of an 1 Ajitsuke Tamago.
1 Ajitsuke Tamago