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1
Combine the flour and salt in a bowl, then add the cold water gradually, mixing until a shaggy dough forms — cold water slows the dough from tightening too quickly, giving you more control during kneading. Knead on an unfloured surface for 8–10 minutes until smooth and firm; working without extra flour keeps the dough's hydration honest, so the final texture is silky rather than stiff. Wrap and rest at room temperature for a minimum of 30 minutes; the dough is ready when it no longer springs back when pressed with a finger, meaning it will roll thinly without shrinking back on itself.
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2
Place the purged clams in a pot with the cold water and bring to a gentle simmer over medium heat, starting from cold so the broth builds flavour gradually rather than shocking the clams and turning the liquor cloudy. Cook just until the clams open, 3–5 minutes, then remove from the heat immediately, because every moment beyond that point toughens the meat and muddies the clean, sweet fragrance you are after. Discard any clams that have not opened, strain the broth through a fine sieve, and reserve the broth and clam meat separately; the broth should smell clean, sweet, and oceanic — not sharp or fishy.
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3
On a generously floured surface, roll the rested dough to a thickness of 1–2 mm; noodles rolled thicker than 2 mm will cook unevenly, leaving the centre doughy while the exterior turns gummy, and the broth will not integrate with them correctly — this is where the dish lives or dies. Fold the sheet in thirds, dusting flour between layers to prevent sticking, then cut across the folds into strips of approximately 5 mm width with a confident, quick stroke, because hesitant cuts produce uneven, ragged noodles. Unfurl the strips and dust lightly with flour to prevent clumping before they go into the pot.
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4
Return the strained clam broth to the pot and bring to a rolling boil before adding the noodles directly into the broth — not into separate water — so the noodles absorb the broth's flavour from the first moment of cooking rather than tasting of plain starch. Cook for 4–5 minutes, stirring occasionally to prevent the noodles from settling and sticking to the base of the pot. The broth will visibly thicken slightly from the noodle starch; this is correct and desirable, and is what gives galguksu its characteristic body.
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5
Return the reserved clam meat to the pot in the final minute of cooking only to warm it through, since the clams are already cooked and any longer in hot broth will see them shrink and toughen. Ladle immediately into bowls and scatter with sliced spring onion. Serve at once, as galguksu sitting in hot broth for 5 minutes or more will continue to cook, becoming soft and over-expanded.