Steps

1. Remove the core from 1/2 onion and slice thinly. [Pro Tip!] You can get creative with the gratin ingredients; using leeks, napa cabbage, or various mushrooms instead of onions also yields delicious results.
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2. Gently separate the roots of 1 pack of shimeji mushrooms and break them apart.
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3. Cut 3 slices of bacon into 1cm pieces. [Pro Tip!] Ham or sausages can be used as a substitute if you don't have bacon.
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4. Devein and peel 8 shrimp. [Pro Tip!] Making a slit along the back makes it easier to remove the vein.
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5. Rub a pinch of salt and a pinch of potato starch into the deveined 8 shrimp. Rinse with water and pat dry thoroughly.
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6. If 8 shrimp are large, cut them into 2-3 pieces.
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7. Boil water in a pot, add about 1% salt, and cook 100g of macaroni for 1 minute longer than the package instructions. [Pro Tip!] Slightly overcooking the macaroni for gratin will result in a more delicious dish.
   🕒 08:07
8. Melt 50g of butter [A] in a frying pan over low to medium heat.
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9. Add 50g of all-purpose flour to the melted butter all at once and stir well over low heat until no dry flour remains. [Pro Tip!] Cook until it resembles custard and small, consistent bubbles appear, being careful not to burn the flour.
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10. Gradually add 300cc of milk to the cooked flour, stirring constantly over low heat to prevent lumps and create a béchamel sauce. [Pro Tip!] Using warmed milk can help prevent lumps, but it's not strictly necessary if you're experienced. Add the milk little by little while stirring.
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11. Set the prepared béchamel sauce aside.
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12. Melt 10g of butter in a separate frying pan. Add 1/2 onion, 1 pack of shimeji mushrooms, 3 slices of bacon, and 8 shrimp. Season with a pinch of salt and a pinch of black pepper, then stir-fry over medium heat. [Pro Tip!] It's generally best to stir-fry the ingredients separately before combining them with the béchamel sauce.
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13. Once the onions are softened and the shrimp are cooked, add 2 tbsp of cooking sake (rice wine) and stir-fry briefly.
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14. Drain the cooked 100g of macaroni in a colander and sprinkle with a pinch of salt while still hot.
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15. Add the stir-fried ingredients (including their juices) and the cooked macaroni to the béchamel sauce and mix well.
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16. Taste and adjust seasoning with a pinch of salt and a pinch of black pepper.
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17. Transfer the mixed ingredients to an oven-safe dish and top generously with shredded cheese (melting type).
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18. Sprinkle breadcrumbs (to taste) over the cheese.
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19. Bake in a preheated oven (or toaster oven) at 230°C (450°F) for about 5 minutes, or until the cheese is melted and browned. [Pro Tip!] If using cooled ingredients, warming them in the microwave before baking will speed up the process.
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20. Once baked, sprinkle with chopped parsley if desired.
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