Steps

1. Remove the eyes from 4 potatoes (420g). After washing, lightly leave some moisture on them and make a cross-shaped cut on the skin. This is the key! Making cuts on the skin makes peeling much easier after cooking. This is a time-saving technique made possible by microwave cooking.
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2. Wrap each potato with the cut facing up individually in plastic wrap, and place them on a heat-resistant plate. This is the key! Wrapping in plastic wrap prevents moisture loss, resulting in moist, non-dry potatoes.
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3. Microwave the potatoes at 600W for 5 minutes.
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4. Cut off the root end of 1/2 onion.
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5. Slice the onion thinly, about 5mm to 1cm wide. This is the key! Thin slicing allows them to cook quickly and brings out their sweetness, preventing the onions from remaining crunchy in the simmered dish.
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6. Add the separated onion slices to a frying pan.
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7. Remove the potatoes from the microwave and flip each one over. This is the key! Flipping them prevents the side touching the plate from becoming hard.
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8. Microwave the potatoes again at 600W for 5 minutes.
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9. Add 200ml water, 2 tbsp Mirin (Sweet Rice Wine), 1 tbsp Soy Sauce, 1 tsp Sugar, and 1.5 tsp Chicken Bouillon Powder to the onions in the frying pan and mix.
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10. Remove the potatoes from the microwave and flip them over again. This is the key! Flipping them again during heating ensures even heat distribution and prevents them from becoming hard.
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11. Pierce a toothpick into a potato. If it slides in easily, they are done. If not, add 1 minute of heating at a time.
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12. Cover the microwaved potatoes with aluminum foil and let them rest for 10 minutes to cook through with residual heat. This is the key! Resting them slowly draws out the potato's sweetness and makes them moist and fluffy. Continuing to microwave can make them dry.
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13. Peel the cooked potatoes starting from the initial cut you made.
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14. Cut the peeled potatoes into large bite-sized pieces. This is the key! Since they will break down a bit when simmering, it's best to cut them larger.
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15. Add the cut potatoes to the frying pan with the simmering liquid and onions.
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16. Heat the frying pan over medium-low heat and simmer until the liquid reduces. Spoon the simmering liquid over the potatoes to let them absorb the flavor. This is the key! Simmering slowly over low heat allows the onions to cook through while the potatoes absorb the flavor.
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17. Once the liquid has reduced, add 15g salted butter and melt it, incorporating it throughout. This is the key! Butter loses flavor if overheated, so adding it just before the liquid is fully reduced maximizes its richness and creaminess.
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18. Continue to simmer until the liquid is no longer watery. The starch from the potatoes will thicken the sauce, and it's ready when it coats the back of a spoon.
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