In a medium bowl, whisk together the sour cream, mayonnaise, white vinegar, sugar, salt, black pepper, garlic powder, and dry mustard until smooth and well combined. This dressing should be creamy and slightly tangy. Set aside at room temperature while you prepare the remaining ingredients.

Bring a large pot of salted water to a boil and cook the rotini pasta according to package directions until al dente. Drain the pasta thoroughly, then rinse it under cold running water until completely cooled. This stops the cooking process and prevents the pasta from becoming mushy. Set the cooled pasta aside in a colander to drain any excess water.

Slice the cucumber into thin 1/8-inch rounds—I prefer to leave the skin on for texture and color, but you can peel it if you prefer. Thinly slice the red onion into rings or half-moons, then place the sliced onion in a bowl of ice-cold water for about 5 minutes. This mellows the onion's sharp bite and makes it crisper. Drain the onion well before adding to the salad.

In a large mixing bowl, combine the cooled pasta from Step 2 with the sliced cucumber and drained onion from Step 3. Pour the dressing mixture from Step 1 over the vegetables and pasta, then add the fresh dill. Toss everything together gently but thoroughly, making sure the dressing coats all the pasta and vegetables evenly. Cover the bowl and refrigerate for at least 40 minutes before serving—this allows the flavors to meld and the pasta to absorb the creamy dressing.