Steps

1. Wash the skin of 1 Grated Yam. Remove any hairy bits if bothersome.
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2. Place the clean Grated Yam into a thick, resealable plastic bag. If using a thin bag, double it up.
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3. Place a cushion like a cloth under the bag with the Grated Yam inside, and gently mash it with a rolling pin. (Here's the trick!) It's easier to mash evenly by lifting the bag occasionally to gather the yam, and changing the spot you mash. Mash larger chunks with your hands.
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4. Transfer the mashed Grated Yam into a heatproof dish, like piping it in. Squeeze out all the contents.
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5. Place 1/2 pod of Mentaiko into a bag with the skin on and mash it. Remove the skin if you dislike its texture.
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6. To the mashed Mentaiko, add 1/2 tbsp Soy Sauce, 3g Bonito Flakes (Katsuobushi), and 20g Pizza Cheese. Mix everything together.
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7. Gently mix the Grated Yam in the heatproof dish with the mixed Mentaiko and other ingredients, incorporating air to make it fluffy. (Here's the trick!) Mixing in chunks of the filling with the smooth parts allows for varied textures.
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8. Generously top the mixed gratin with 30g Pizza Cheese.
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9. Scatter 5g Butter in a few spots, if desired. (Here's the trick!) Adding butter enhances the richness and flavor.
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10. Place in a toaster and bake until a golden brown color is achieved. (Here's the trick!) If you want a beautiful color, adjust by moving the dish around during baking.
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11. Top the baked gratin with chopped green onions (as needed), bonito flakes (Katsuobushi) (as needed), and shredded nori (as needed), according to your preference.
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