Place the beef tenderloin in a bowl of cold water and let it soak for 10 minutes to drain out any blood. This step helps prevent any gamey flavor and keeps the dish clean and savory.
If your quail eggs aren’t pre-boiled, place them in a pot of cold water and bring it to a boil. Once boiling, cook for 10 minutes. Transfer the eggs to a bowl of cold water immediately to stop the cooking process and make them easier to peel.
Carefully peel the eggs to avoid exposing the yolks. If any eggs break and the yolk shows, consider removing them to prevent clouding the sauce later. Set the peeled eggs aside.
In a large pot, add enough water to fully cover the beef. Add the beef and boil for 10 minutes. Skim off any foam or impurities floating on the surface to keep the broth clear.
Remove the beef from the pot and cut it into 1-2 cm thick chunks. To avoid leftover blood residue, wash the pot before continuing with the braising process.
In the clean pot, add water, soy sauce, sugar, and mirin. Stir well to dissolve the sugar. Add the beef chunks and quail eggs to the pot. Bring the mixture to a boil over medium-high heat.
Once boiling, reduce the heat to low and simmer for 50 minutes, allowing the flavors to fully infuse the beef and eggs. The sauce will gradually reduce, creating a rich and glossy coating.
Transfer the braised beef and quail eggs to a serving dish. If the beef chunks are too large, slice them into thinner pieces for easier serving. Serve warm with steamed rice or alongside other Korean side dishes (banchan).