Add the sliced cucumbers to a colander and sprinkle with about 1/4 teaspoon salt. Toss gently and let them sit for 7 to 10 minutes, just until they release a little water but still stay crisp.
Spread the cucumbers over paper towels and gently pat them dry. This helps keep the creamy cucumber salad from turning watery.
In a large bowl, mix the sour cream, mayonnaise, fresh dill, red onion, garlic, lemon juice, sugar, black pepper, and the remaining 1/4 teaspoon salt until smooth and creamy.
Add the drained cucumbers to the bowl and gently fold everything together until the cucumber slices are coated in the creamy dressing.
Cover and chill for 15 to 30 minutes before serving. Stir gently, taste for seasoning, and serve cold with extra fresh dill if desired.