In a small bowl, combine sour cream, lemon juice, garlic, dill, salt and pepper. Refrigerate until ready to use.
Slice cucumbers (I use a mandoline) into thin rounds. Place into a large mixing bowl. Add thinly sliced onion. Just before serving, add dressing. Gently toss to coat cucumbers and onions. Salad will stay fresh for about 2-3 hours if refrigerated.