Slice cucumbers into 1/4-inch rounds. Place in a colander, sprinkle with 1 teaspoon salt, toss, and let drain for 15 minutes. Pat dry with paper towels.
Thinly slice red onion into half-rings. Soak in cold water for 5 minutes to mellow the bite, then drain and pat dry.
Finely chop dill, chives, and parsley.
In a large bowl, whisk together sour cream, mayonnaise, ranch seasoning, buttermilk, garlic, onion powder, and black pepper until smooth.
Add cucumbers, red onion, and herbs to the dressing. Gently fold until everything is coated.
Cover and refrigerate for at least 30 minutes (or up to 4 hours) to let flavors meld.
Before serving, give it a stir and adjust salt if needed. Sprinkle with feta or cotija if using.