Steps

1. Finely chop the onion.
   🕒 01:45
2. In a bowl, combine ground chicken thighs 250g, [A] sugar 1 tsp, salt 1/2 tsp, potato starch 2 tsp, grated ginger 1/2 tsp, panko breadcrumbs 50g, pepper a pinch, and egg 1. Mix well by hand until sticky. (Tip: Thoroughly kneading ground meat dishes until sticky improves texture. Using more panko creates a chewy texture.)
   🕒 02:46
3. Add the finely chopped onion 1/2 and mix briefly. (Tip: Overmixing onions releases moisture, so add them last and mix quickly.)
   🕒 03:35
4. If time permits, refrigerate the mixture for about 30 minutes. (Tip: Chilling the mixture allows the flavors to meld, resulting in a smoother texture.)
   🕒 04:03
5. Form the meatball mixture into bite-sized balls. (Tip: Using a spoon to scoop and shape helps create uniform balls.)
   🕒 04:24
6. Break apart the shimeji mushrooms. If the stems are thick, tear them by hand. (Tip: Breaking from the base minimizes waste. You can break them by hand without a knife.)
   🕒 05:54
7. Finely slice the Japanese bunching onions.
   🕒 06:45
8. In a bowl, combine [C] mayonnaise 3 tbsp, soy sauce 1/2 tsp, sugar 2 pinches, and black pepper a pinch to create the tuna mayo base.
   🕒 07:02
9. Add the drained canned tuna (about half the oil removed) to the tuna mayo base and mix.
   🕒 07:13
10. Boil water in a pot, add the separated shimeji mushrooms 1 pack, and boil for 1 minute. Drain.
    🕒 07:53
11. In another pot, boil water and add 2 eggs. Boil for 10 minutes for hard-boiled eggs. (Tip: Stirring for the first minute while boiling centers the yolk. Hard-boiled eggs are recommended for bento as soft-boiled eggs spoil easily.)
    🕒 08:13
12. Heat vegetable oil 1 tbsp in a frying pan over medium heat. Roll the meatballs in the pan until browned (approx. 5 minutes). They don't need to be cooked through completely at this stage.
    🕒 08:55
13. Wipe off any excess oil from the frying pan.
    🕒 09:36
14. Add [B] water 100cc, tomato juice 200cc, soy sauce 2 tbsp, mirin (sweet rice wine) 2 tbsp, sugar 1 tbsp, and vinegar 1 tsp to the frying pan. Bring to a boil.
    🕒 09:40
15. Cover and simmer over low heat for 3 minutes, allowing the meatballs to cook through completely. (Tip: Simmering on low heat prevents the sauce from reducing too much and the meat from drying out.)
    🕒 10:15
16. Thoroughly pat dry the boiled shimeji mushrooms with paper towels.
    🕒 10:38
17. Add the shimeji mushrooms, sliced Japanese bunching onions 2 stalks, and salt to taste to the tuna mayo base and mix.
    🕒 10:45
18. Plunge the boiled eggs into an ice bath to cool. (Tip: Cooling in an ice bath makes the shells easier to peel.)
    🕒 11:31
19. Add the slurry of potato starch and water 1 tbsp to the meatball frying pan and stir until thickened. Bring to a boil. (Tip: Boiling after thickening helps the sauce maintain its consistency.)
    🕒 11:56
20. Peel the cooled eggs. Use a thread to cut them cleanly in half. (Tip: Using thread allows for a neat, even cut.)
    🕒 13:08
21. Top the cut egg yolks with seaweed paste (nori tsukudani) to taste and white sesame seeds a pinch.
    🕒 13:38
22. Pack rice into the bento box, line with lettuce (if using), and arrange the meatballs and the two side dishes.
    🕒 14:18