In a small bowl, mix together the sour cream, lemon juice, garlic, dill, salt, and pepper. Stir until well combined and set aside to let the flavors meld.
Peel the cucumbers if necessary, then slice them into thin rounds or half-rounds if using larger cucumbers.
In a large mixing bowl, combine the sliced cucumbers with the thinly sliced onion.
Just before serving, pour the prepared dressing over the cucumber and onion mixture. Stir gently to coat all the vegetables evenly.
Refrigerate the salad to let the flavors blend more, ideally for about 2–3 hours.
Notes
Use fresh dill for the best flavor.
Substitute Greek yogurt for sour cream for a lighter dressing.
Let the salad rest for at least an hour before serving for maximum flavor.