Cook the pasta in well salted water for 2 minutes longer than the packet suggests. Once cooked, drain and quickly cool the pasta.: When the water is at a full rolling boil you will notice steam and a steady, lively bubbling sound, and adding a generous pinch of salt seasons the pasta from the inside out; cooking it slightly past the packet instructions helps ensure the texture does not go chalky after chilling, because the pasta will continue to soften as it cools; one sign you are done is when a piece pulls apart easily yet still has a slight tooth; avoid under salting the water, and beware of overcooking which yields a mushy final salad.
Mix the mayonnaise and sour cream together in a small bowl, season with salt and pepper.: Drain the pasta promptly so it stops cooking in the residual heat, and then run cool water over it or spread it on a tray to stop steam; you will feel the temperature drop and see the surface go matte instead of glossy; rapid cooling preserves a firm bite, and it prevents the dressing from thinning out from heat; a common mistake is letting the pasta sit steaming in the colander, which leads to an over soft texture.
Pour the dressing over the pasta and stir gently to coat the pasta.: Whisk until smooth so the dressing emulsifies and has a uniform pale sheen; smell the mixture for freshness and taste to balance saltiness and tang; a well emulsified dressing clings to the pasta rather than pooling; if the dressing seems too thick, a teaspoon of lemon juice can loosen it gently; do not over salt here because the smoked salmon will contribute sodium later.
Add the chopped celery, gherkins, dill and chives to the pasta and then fold together to combine all the ingredients.: As you combine, notice the dressing adhering to the pasta curves and pooling in crevices, which signals even coating; use a folding motion to avoid crushing shapes and to keep the strands separate; the sound is a soft, gentle movement rather than vigorous sloshing; stirring too aggressively breaks the pasta and releases starch, making the salad gluey, so be gentle.
Add the smoked salmon pieces and stir well.: When these are folded in, watch for little bright green flecks and tiny specks of gherkin distributed across the bowl, which shows even distribution; the crunch of celery should contrast the pasta texture; this step layers flavors so they appear in every bite; a mistake to avoid is adding large uncut pieces that create uneven flavor pockets.
Grate in 1 tablespoon of lemon zest (save the lemon for a G&T later).: Tear or chop the smoked salmon into bite size ribbons and fold them through so they nestle among the pasta shapes, releasing a faint smoky aroma; the salmon should remain in soft ribbons rather than turning to mush; this ensures pockets of savory richness; avoid overworking the fish, which can turn it pasty and lose its attractive texture.
Garnish with extra dill and chives and serve, or refrigerate for up to 24 hours.: Freshly zested lemon oils will perfume the salad, producing a bright citrus aroma that lifts the cream and accentuates the smoked salmon ; zest just above the bowl so the fragrant oils mingle with other ingredients; a warning is to zest only the yellow skin and avoid the bitter white pith beneath, which would introduce an unwanted bitterness.
Garnish with extra dill and chives and serve, or refrigerate for up to 24 hours: Sprinkle more chopped herbs on top so the presentation looks fresh and fragrant, and if you refrigerate you will notice the salad firm slightly as flavors meld; chilling also calms the sharpest edges so the salad tastes more cohesive; when serving from chilled, give it a gentle toss and taste for seasoning since cold dulls flavors and may need a tiny extra squeeze of lemon or more pepper; avoid storing beyond 24 hours, as the pasta texture and herb brightness will decline.