Rinse the clams under cold running water and scrub the shells well. Discard any clams with cracked shells. If any are open, tap them firmly on the counter: if they do not close, discard them. If the clams are sandy, place them in a bowl of cold salted water for 30–60 minutes to help them purge grit, then lift them out, rinse again, and keep them cold. The entire dish depends on clean clams. Sandy clams will ruin the sauce no matter how well the pasta is cooked. That upfront cleaning and purging stage is the genuine foundation the entire dish rests on — no amount of careful pasta cooking or sauce building later can rescue a dish undermined by grit at its very source.

Bring a large pot of water to a boil. Salt it, but slightly less heavily than for a normal pasta dish because the clam liquor will season the sauce later. Do not cook the spaghetti yet — first open the clams and build the sauce base. That deliberately lighter salting accounts for real, additional seasoning still to come from the clams' own naturally salty liquor, protecting the finished dish from turning oversalted once everything comes together.

In a wide deep skillet or sauté pan large enough to later hold the pasta, heat the olive oil over medium heat. Add the sliced garlic and chili flakes and cook gently for 30–60 seconds, just until fragrant. Do not let the garlic brown deeply or it will turn bitter. Add the clams and pour in the white wine. Immediately cover with a lid and cook for 2–5 minutes, shaking the pan once or twice, until the clams begin to open. As the clams open, transfer them with tongs to a bowl. Continue cooking only until nearly all are open. Discard any clams that remain shut after cooking. Do not overcook the clams. As soon as they open, remove them. Overcooked clams turn shrunken and rubbery. Pulling each clam the moment it opens, rather than waiting for the entire batch to finish together, protects the earliest-opening pieces from sitting over continued heat considerably longer than they actually need.

Pour the cooking liquid left in the pan through a fine-mesh sieve lined with a paper towel, cheesecloth, or coffee filter into a bowl or jug to catch any sand. Wipe out the pan if needed. Never skip straining the clam liquor. Even well-cleaned clams often release some grit. That straining step is a genuine, final safeguard against exactly the kind of residual sediment even a thorough purge doesn't always fully eliminate, protecting the finished sauce from any unwelcome grit.

Drop the spaghetti into the boiling water and cook until very al dente, usually 2–3 minutes less than fully done according to the package. Before draining, reserve at least 250ml pasta water. Pulling the spaghetti genuinely short of al dente leaves real room for it to finish cooking directly in the pan, absorbing the clam liquor and building the sauce's own emulsion as it does.

Return the cleaned pan to medium heat. Pour in the strained clam liquor. Let it simmer briefly for 30–60 seconds to concentrate slightly, but do not reduce it aggressively. Add a small splash of reserved pasta water if needed to balance the salinity and create enough liquid to finish the pasta. Taste the liquid before adding any salt. Often it needs none. That deliberately brief simmer concentrates the liquor's own flavour just slightly without pushing its natural salinity any further than it already carries on its own.

Transfer the very al dente spaghetti directly into the pan with the clam liquor. Toss and stir continuously over medium heat, adding reserved pasta water a little at a time until the sauce becomes glossy and lightly emulsified. When the spaghetti is nearly done and the sauce is clinging to the strands rather than pooling thinly in the pan, add the chopped parsley and lemon zest. Return the clams to the pan and toss gently just long enough to warm them through, about 30–45 seconds. The sauce should be light, glossy, and fluid — not watery, not oily, and not thick like a cream sauce. The emulsion comes from the clam liquor, olive oil, and starchy pasta water. Returning the clams only at this final stage, for the briefest possible warming, protects them from any further cooking that would push their already-tender texture toward rubbery.

Twirl the spaghetti into warm bowls and distribute the clams evenly. Spoon over any remaining pan juices. Finish with a last drizzle of good olive oil if desired. Serve immediately. This dish's whole appeal depends on that fluid, glossy emulsion staying intact — it continues tightening and changing the longer it sits, which is exactly why serving without delay matters here.