Steps

1. Pierce the beef block all over with a fork.
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2. Season the entire surface of the beef with salt. Wrap in plastic wrap and let it sit at room temperature for about 1 hour. [Key Tip!] Salting and resting the meat enhances its vibrant color. If you're short on time, you can sear it immediately.
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3. Thinly slice the onion. [Key Tip!] The onion is simmered with the meat to infuse flavor, so the cut doesn't need to be precise. You can substitute with green onions, garlic, or ginger.
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4. Heat 1 tbsp of vegetable oil in a frying pan over medium-high heat and sear all sides of the beef until browned. The goal is to sear the surface and seal in the juices, not to cook it through.
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5. In a pot, combine 600cc of water, 100cc of soy sauce, 100cc of Mirin (Sweet Rice Wine), 100cc of Cooking Sake (Rice Wine), 1 tbsp of sugar, the thinly sliced onion, and 5g of kombu (kelp).
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6. Bring the pot to a boil to evaporate the alcohol. Allow the flavors of the onion and kelp to infuse into the liquid.
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7. Reduce the heat to low, add the seared beef, and simmer for 3 minutes. [Key Tip!] This is an adaptation of Japanese duck cuisine. Simmering in the sauce steams the meat, allowing flavors to penetrate and extending its shelf life. Adjust simmering time according to the size of the meat (400-500g).
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8. Flip the meat and simmer for another 3 minutes. [Key Tip!] Cooking time differs on each side, so flipping ensures even cooking. The simmering liquid can be used to make seasoned boiled eggs, or to cook potatoes and tofu.
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9. Turn off the heat and place paper towels over the beef, ensuring the liquid covers it.
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10. Let it cool to room temperature, allowing the residual heat to cook the meat and the flavors to meld (approximately 1 hour). Once cooled, transfer to a container and refrigerate, keeping it submerged in the simmering liquid. [Key Tip!] Storing the meat in the liquid prolongs its freshness. The flavor improves if eaten the day after preparation.
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11. Take a portion of the cooled roast beef simmering liquid into a separate small pot.
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12. Bring the liquid to a simmer and thicken to your desired consistency using potato starch slurry to create a sauce. It will thicken as it cools, so aim for a slightly looser consistency.
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13. Slice the cooled roast beef into desired thickness.
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14. Arrange the roast beef on a plate and garnish with watercress, cherry tomatoes, the simmered onion, and whole grain mustard.
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15. Pour the prepared sauce over the roast beef.
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