Steps

1. Take the Beef Round Roast (whole) 300g out of the refrigerator and bring it to room temperature. (Key Tip!) Allow about 1 hour in winter and 30 minutes in summer.
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2. Pat the surface of the room-temperature beef dry with paper towels. (Key Tip!) Thin cuts will cook through too quickly, so choose a cut that is thick in length.
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3. Rub Salt 3g all over the beef.
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4. Sprinkle Black Pepper (to taste) all over the beef.
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5. Heat Olive Oil 1 tbsp in a frying pan over medium heat. Spread the oil evenly and warm for about 30 seconds.
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6. Sear the beef in the heated frying pan. Aim for 1 to 1.5 minutes per side over medium heat until browned all over. (Key Tip!) Browning the entire surface seals in the meat's juices and creates a fragrant finish. Be careful of oil splattering to avoid burns.
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7. Once browned all over, turn off the heat.
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8. Wrap the seared beef tightly in aluminum foil, doubling it up. (Key Tip!) Wrap immediately after searing.
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9. Place the aluminum-foiled beef into a heat-resistant plastic bag or zip-top bag for sous vide. (Key Tip!) Always check the heat resistance temperature indicated on the bag.
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10. Fill a pot with plenty of water and submerge the bag containing the beef. (Key Tip!) Ensure all air is removed from the bag. If not, the meat will not cook evenly.
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11. Place the pot over heat and bring it just to a simmer (about 70-80°C). You'll see many small bubbles forming at the bottom and sides of the pot.
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12. Once it's just before boiling, reduce the heat to very low. Place a heat-resistant plate at the bottom of the pot and then place the bagged beef on top. (Key Tip!) Always use a plate to prevent the plastic bag from directly touching the bottom of the pot.
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13. If the beef floats, place another heat-resistant plate or a lid on top to keep it submerged. Simmer for 5 minutes. (Key Tip!) Maintain a very low heat to avoid boiling. Boiling will cause the meat to lose moisture and result in a dry texture.
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14. After 5 minutes, turn off the heat and let it rest for 15 minutes to cook through with residual heat. (Key Tip!) Residual heat cooking will result in moist and tender meat.
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15. After 15 minutes, remove the beef from the plastic bag and let it cool down. (Key Tip!) Be careful as it will be hot.
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16. Rest the cooled beef in the refrigerator for 1 hour.
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17. Make the sauce. Grate the Onion 50g.
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18. Prepare the frying pan used to cook the meat.
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19. Add [A] Soy Sauce 2 tbsp, [A] Cooking Sake (Rice Wine) 2 tbsp, [A] Sugar 1 tbsp, [A] Vinegar 1 tbsp, [A] Lemon Juice 1/2 tsp, and the grated Onion to the frying pan.
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20. Mix the sauce ingredients well and bring to a boil over medium heat. Stir occasionally.
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21. Once boiling, reduce the heat to medium-low to medium and simmer for about 2 minutes. (Key Tip!) Add the meat juices remaining in the pan from searing the roast beef to the sauce for extra flavor.
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22. After 2 minutes, turn off the heat and let the onion sauce cool.
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23. Remove the roast beef, which has been resting in the refrigerator for 1 hour, from the aluminum foil.
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24. Slice the roast beef as thinly as possible.
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25. Serve the sliced roast beef with the cooled onion sauce poured over it.
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