Slice the 5 cucumbers into thin rounds. A sharp knife works, but a mandoline gives the most consistent results.
Place the sliced cucumbers in a large bowl and toss with 1 teaspoon kosher salt. Let them sit for 10-15 minutes. You'll see water pooling at the bottom of the bowl - that's exactly what you want.
Drain the cucumbers well, then gently press or pat them dry with a paper towel. Don't skip this step.
In a separate bowl, whisk together ¼ cup sour cream and ¼ cup Greek yogurt until smooth.
Add 3 tablespoons fresh dill, 1 tablespoon lemon juice (or 1 to 2 teaspoons white vinegar), ¼ teaspoon black pepper, ¼ teaspoon white pepper and a tiny pinch of sugar if you'd like. Stir until combined.
If you want a slightly looser dressing, stir in a small splash of milk. This is optional.
Taste the dressing and adjust. Add more acid if it needs brightness, more salt if it's flat.
Add the drained cucumbers to the dressing and toss gently until everything is coated.
Chill for 10 minutes before serving, or serve right away.
Notes
Salt the cucumbers and actually wait. This step removes excess water from the cucumbers so your dressing stays creamy.Slice thin. Very thin. A mandoline makes this easy and consistent.