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1
Place the beef bones in a large pot, cover with cold water, and bring to a boil, then drain and rinse the bones and pot, discarding the grey water, so that the finished stock is clear and clean-tasting rather than cloudy and sour. Return the bones and the whole brisket or flank to the clean pot, cover with 3 litres of cold water, and bring to a boil. Reduce to a sustained simmer and cook until the beef is fully tender and yielding, approximately 1½–2 hours — the beef is ready when a chopstick passes through the thickest part with no resistance.
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2
Lift the beef from the stock and set it aside, allowing it to cool until it is warm to the touch — not hot, not cold — because warm meat separates cleanly along its muscle fibres into long, fine threads, while cold meat tears into chunks and hot meat shreds unpredictably. This is where the dish lives or dies: the texture of those threads is the spine of the soup. Working with your fingers, pull the beef along the grain into the thinnest threads possible — fine enough to tangle around a chopstick, substantial enough to provide a distinct bite.
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3
Transfer the shredded beef to a bowl and add 4 tbsp gochugaru, 2 tbsp sesame oil, and the minced garlic, then mix thoroughly until every strand is coated, so the beef carries heat and fragrance within itself rather than acting as a bland addition suspended in a spiced broth. Work the seasoning in with your hands so the oil and pepper penetrate the threads rather than sitting on their surface. Set aside.
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4
Strain the bone stock and return it to the pot, then in a separate small pan over medium heat warm 2 tbsp sesame oil, add 3 tbsp gochugaru, and fry, stirring constantly, for 30 seconds, so the pepper blooms in the fat and turns the oil a deep, uniform red with a toasted, peppery fragrance rather than the raw, dusty heat it would otherwise contribute. Watch it closely — the oil should smell toasted and peppery, not burnt. Add the strained beef stock to the bloomed gochugaru oil and bring to a simmer.
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5
Add the soaked and drained gosari and the green onion lengths to the simmering broth and cook for 20–25 minutes until the gosari is tender and the green onions are completely soft, giving them enough time to release their sweetness into the broth and lose any raw, grassy edge. Season with soy sauce and salt. The broth should be deeply red, intensely spicy, and carry a silky body from the gelatin of the bone stock — not thin or watery.
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6
Add the seasoned shredded beef to the simmering broth and stir to integrate, then simmer for a further 5–10 minutes so the beef threads fully uncurl, absorb the broth, and distribute evenly rather than clumping together. Taste and adjust salt. Ladle into deep bowls and serve immediately with plain white rice in a separate bowl alongside.