Steps

1. In a bowl, combine Cooking Sake (Rice Wine) 2 tbsp, Mirin (Sweet Rice Wine) 1 tbsp, and Soy Sauce 2 tsp.
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2. Mix everything together to create the sauce mixture.
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3. Prepare Butter 15g.
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4. Wash the dirt off the surface of Japanese Yam 400g with water using aluminum foil, and remove the hairy roots. The skin is thin, not bitter, and doesn't negatively affect the texture, so you can eat it whole by just removing the hairy roots. Be careful not to rub too hard, as this can peel the skin and make it sticky.
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5. Cut the Japanese yam into small, bite-sized pieces. Cutting them as small as possible will make the subsequent steps easier.
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6. Place the cut Japanese yam into a zip-top bag.
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7. Use a rolling pin or a stick to pound the Japanese yam in the bag until finely mashed. Pound thoroughly, even after flipping. Thoroughly pounding it until you think you can't pound it any more will allow you to enjoy both creamy and fluffy textures. It's easier and tastier than grating.
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8. Place Natto 2 packs on a cutting board. Set aside the included sauce for later use.
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9. Finely chop the natto with a knife. If the beans are too large, they won't mix well with the Japanese yam, so chop them finely. You can substitute with pre-mashed natto, but pounding it helps the flavor distribute evenly and retain the natto taste.
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10. Add the included sauce from 2 packs to the mashed natto and mix until sticky.
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11. Heat a frying pan over low heat and add a generous amount of oil.
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12. Once the oil is heated, pour the pounded Japanese yam into the frying pan.
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13. Sprinkle with Salt 2 pinches and Pepper to taste, and mix well. Adding a bit more pepper than you think you need will give it a nice punch.
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14. Stir and cook the Japanese yam slowly until the stickiness reduces and it becomes fluffy.
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15. Spread out the Japanese yam and cook for about 2 minutes until browned.
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16. Flip the mixture, spread it out again, and cook for another 2 minutes until browned. Cooking slowly allows it to brown and develop flavor. It's ready when most of the stickiness is gone and it has a creamy yet fluffy texture.
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17. Add the prepared sauce mixture to the Japanese yam. Seasoning the Japanese yam and natto separately is the key to deliciousness.
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18. Turn the heat to medium and stir-fry briefly until the alcohol evaporates and the liquid reduces.
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19. Once the liquid is gone, turn off the heat and add the Butter, tossing to coat with residual heat. Turning off the heat maximizes the butter's flavor.
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20. Once the butter has melted, the fried Japanese yam is complete.
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21. Briefly mix the cooked Japanese yam with the pre-prepared natto.
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22. Transfer to a serving dish and top generously with green onions and seaweed, if desired. By finely mashing the Japanese yam, it becomes easier to eat with the natto and creates a harmonious blend in your mouth. If the taste is too light, adjust with light soy sauce.
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