Steps

1. Finely mince 1/8 onion after thinly slicing it.
   🕒 01:44
2. Place the minced onion in a bowl, mix well with Mentsuyu (Noodle Soup Base) 1/2 tsp and Honey 1/3 tsp, and let it soak for about 5 to 10 minutes. [Key Tip!] This removes the onion's harshness and adds richness and umami to the tuna mayo.
   🕒 02:12
3. Core and thinly shred 80g of cabbage into slightly thick strips.
   🕒 02:35
4. Place the shredded cabbage in a heatproof bowl, cover loosely with plastic wrap, and microwave for 30 seconds at 600W.
   🕒 03:00
5. After microwaving, remove the plastic wrap to let steam escape, then cover again and let it cool to room temperature. [Key Tip!] The cabbage will become tender and easier to sandwich, but it will retain some crispness.
   🕒 03:18
6. Thoroughly drain the oil from 1 can of canned tuna. [Key Tip!] If using oil-packed tuna, drain the oil well; if using water-packed, drain the water thoroughly.
   🕒 03:33
7. Add the drained tuna to the bowl with the soaked onion, and mix well with Mayonnaise 1 tbsp to create the tuna mayo.
   🕒 03:47
8. Thoroughly pat dry any moisture from the cooled cabbage. [Key Tip!] Always wipe away moisture before assembling the sandwich.
   🕒 04:05
9. Spread about 5g of salted butter on the inside of each of the 2 slices of bread. [Key Tip!] Buttering prevents the sandwich filling's moisture from soaking into the bread. It's easier to spread if the butter is softened at room temperature or warmed in the microwave for about 10 seconds. Mayonnaise can also be used as a substitute.
   🕒 04:18
10. On one slice of buttered bread, place 1 slice of melting cheese.
    🕒 04:52
11. Spread the tuna mayo on top of the cheese, mounding it slightly in the center.
    🕒 04:54
12. Place the drained cabbage on top of the tuna mayo and gently press down with your hands.
    🕒 05:07
13. Top the cabbage with another slice of melting cheese, then cover with the remaining slice of bread and press down lightly.
    🕒 05:13
14. Heat about 5g of salted butter in a frying pan over medium heat, letting it melt and spread evenly.
    🕒 05:24
15. Once the butter has melted, reduce the heat to low, place the sandwich in the pan, and cook for about 2 minutes, pressing down gently, until golden brown. [Key Tip!] Lightly pressing with a spatula while cooking helps maintain the sandwich's shape.
    🕒 05:42
16. Once a good golden-brown color appears on one side, flip the sandwich. Add the remaining 5g of salted butter, let it melt, and spread it onto the bread. Cook the second side for 1 to 2 minutes until golden brown. [Key Tip!] The second side cooks faster, so it's best to cook it for a slightly shorter time.
    🕒 06:08
17. Remove from the pan once both sides are nicely browned. Cut in half and serve.
    🕒 06:37