Start by rinsing the Beef Leg Bones and the Beef Brisket under cold water to remove surface blood and bone fragments; place them in a large stockpot and cover with cold Water.
Bring the pot to a rolling boil over high heat and boil for 10–15 minutes to blanch and release impurities; drain and thoroughly rinse the bones, brisket, and the pot to remove scum.
Return the cleaned bones and brisket to the pot and fill with fresh cold water Water to cover by several inches. Bring to a boil, then reduce to a very gentle simmer and cook uncovered for 6–10 hours, skimming any foam occasionally; the long, gentle simmer is what produces the milky white broth.
After 2–3 hours you can remove the brisket when tender but still sliceable; transfer it to a plate to cool. Continue simmering the bones alone until the broth is deeply flavored and opaque.
When the broth reaches the desired color and richness, remove the bones and strain the liquid through a fine-mesh sieve into a clean pot to remove any remaining solids; skim off excess fat if desired.
Thinly slice the cooled brisket against the grain and return the slices to the strained broth so they warm through and soak up flavor.
Cook the Somyeon in a separate pot of boiling water according to package instructions, drain, and rinse briefly under warm water to prevent sticking.
Prepare the garnishes: thinly slice the Scallions and finely mince the Garlic.
To serve, place a scoop of Cooked Short-Grain Rice or a nest of somyeon into each bowl, ladle the hot seolleongtang over it, add several slices of brisket, and season each bowl to taste with Sea Salt and freshly ground Black Pepper. Top with scallions and a bit of raw minced garlic for brightness.
Store leftover broth and meat in the refrigerator for up to 3 days or freeze for longer keeping; reheat gently and adjust seasonings before serving.
