Bring a pot of salted water to a boil and cook the rotini pasta until al dente according to package directions, then drain and rinse thoroughly with cold water to stop the cooking and remove excess starch. While the pasta cooks, halve the cucumber lengthwise, use a spoon to scoop out and discard the seeds, then slice into half-moons. Thinly slice the red onion and immediately submerge it in a bowl of cold water—this mellows its sharp bite and makes it crisper. Set all prepared vegetables aside.

In a medium bowl, whisk together the sour cream, mayonnaise, white vinegar, sugar, salt, black pepper, and garlic powder until smooth and well combined. I like to add the lemon zest at this point so it infuses into the dressing and becomes more aromatic. Fold in the fresh dill gently to preserve its texture and vibrant color. Taste and adjust seasoning if needed—the dressing should be tangy, slightly sweet, and herbaceous.

Drain the soaked red onion well and add it to a large serving bowl along with the cooked pasta from Step 1 and the sliced cucumber. Pour the creamy dressing from Step 2 over everything and gently toss until all components are evenly coated. I've found that letting this salad sit for 30–40 minutes before serving allows the flavors to meld and the pasta to absorb the dressing, creating a more cohesive dish. Cover and refrigerate until ready to serve, stirring occasionally if you're checking on it.