Heat a frying pan over medium-high heat until hot. Add the oil and then add the tofu and 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.
Add the onions along with the sake, mirin, soy sauce, and 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.
Add the 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.
To pack the bento, add a layer of rice to the bottom of the bento box and top the right 2/3 with the Gyunabe.
Add some benishōga to the top left edge of the box and then stagger some tofu on top of it.
Layer some of the Negi on top of the tofu, and then finish your Demon Slayer bento box off with half of an Ajitsuke Tamago.