Steps

1. Cut off the root of 1/2 onion and make slits on both sides.
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2. Imagine keeping the base of the onion intact, make diagonal cuts from the edge and slice with the entire knife.
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3. Once you reach the end, turn it sideways and make about 3 horizontal cuts (partially is fine).
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4. Once slitted, finely chop from the edge into a mince.
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5. When you reach the slitted part, make cuts again and chop finely as well.
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6. If you want it even finer, press down on the tip of the knife, lift the base, and chop finely by tapping.
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7. Transfer the minced onion to a heatproof container, add 2 pinches of salt, and mix well.
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8. Heat in the microwave for about 1 minute without covering. (This is the secret! Heating the onion turns its pungency into sweetness, leaving a crisp texture while removing excess moisture.)
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9. Let it sit for about 5 minutes to draw out excess moisture.
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10. In a separate bowl, combine 3 tbsp Mayonnaise, 2 tsp Soy Sauce, 1 tsp Lemon Juice, and 1 tsp Sugar for the dressing.
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11. In a separate container, mix 1.5 tbsp Mayonnaise and 5g Mustard to make mustard mayonnaise.
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12. Slightly open the tuna cans and add the oil and liquid from 2 cans of tuna to the bowl with the dressing from Step 10. (This is the secret! Don't mash the tuna too much, as you'll lose the flavorful liquid; only add the liquid.)
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13. Whisk until the oil and liquid are combined and the sauce becomes thick. (This is the secret! A thick sauce prevents it from soaking into the bread and becoming soggy.)
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14. Once the sauce is thick, add the tuna to the bowl and gently mix without mashing. (This is the secret! If you use a whisk after adding the tuna, the seasonings won't mix well, so make sure the sauce is properly thickened first.)
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15. Line a tray with two layers of paper towels and spread out the onion from Step 9.
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16. Cover the onion with paper towels and press down lightly with your body weight to thoroughly remove moisture. (This is the secret! Don't press too hard, or you'll lose the texture.)
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17. Add the drained onion to the tuna bowl and gently mix without crushing the onion.
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18. Arrange 4 slices of 8-slice-thick bread on a cutting board.
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19. Spread the mustard mayonnaise made in Step 11 to the edges of the bread. (This is the secret! Using mustard mayonnaise instead of butter seasons the bread and creates an oil barrier between the bread and filling, making it less likely to become soggy.)
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20. Divide the tuna into 4 portions, place them on the bread, and spread them so the center is slightly mounded. (This is the secret! By putting more filling in the center and less towards the edges, the cut surface looks beautiful and the filling is less likely to spill out.)
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21. Place the other slices of bread on top.
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22. Wrap the entire sandwich tightly with plastic wrap (don't press too hard to the point where the filling squeezes out).
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23. While still wrapped, chill in the refrigerator for about 15 minutes to allow the bread and filling to meld.
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24. Remove from the refrigerator and cut in half over the plastic wrap to prevent the tuna from spilling out.
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25. Sprinkle with dried parsley or black pepper, if desired.
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