Steps

1. Finely slice 1/8 onion, then chop into coarse mince. Do not soak in water; let it air out to reduce sharpness.
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2. Thinly slice 1/2 cucumber.
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3. Thoroughly pat dry any excess moisture from the thinly sliced cucumber with paper towels. Since it releases a lot of water, drain as much as possible.
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4. Slightly open the lid of 1 small can (70g) of tuna and lightly press to drain excess oil. This prevents it from becoming too oily.
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5. Place the drained tuna in a bowl and lightly flake it until no large chunks remain. This is key to eliminating a fishy texture.
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6. Add the air-exposed minced onion to the flaked tuna.
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7. In the bowl, add 1-2 tbsp mayonnaise, 1 tsp soy sauce, 1 tsp vinegar, and generous black pepper, then mix briefly.
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8. Taste and adjust by adding more mayonnaise little by little if the texture isn't smooth enough or if you still notice any fishiness.
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9. Tear 1-2 slices of cheese into small pieces and add them to the tuna bowl.
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10. Lightly heat the tuna with cheese in a microwave or similar until the cheese begins to melt, then immediately remove.
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11. Quickly mix to ensure the melted cheese is well incorporated with the tuna. This is the trick! Adding cheese increases richness and umami, resulting in a smoother finish. Use regular sliced cheese, not melting cheese.
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12. On one side of 2 slices of 8-slice-cut sandwich bread, spread a mixture of mayonnaise and tube wasabi. The wasabi pairs well with tuna.
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13. On top of the bread with mayo-wasabi, generously layer the drained cucumber slices, overlapping them slightly like avocado slices. Overlapping the cucumber helps prevent it from falling apart.
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14. On the other slice of bread, generously spread the prepared tuna mixture. No need to spread mayonnaise or anything else on this slice.
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15. Place the tuna-covered bread on top of the cucumber-covered bread to form a sandwich.
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16. Tightly wrap the sandwich in plastic wrap and let it sit for a moment to allow the flavors to meld. This will help it stabilize and make it easier to cut.
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17. While still wrapped, cut into your desired size. You can trim the crusts beforehand if preferred.
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