How to Make It
Preheat grill to medium (about 400°F). Bring the potatoes, 1/4 cup of the salt, and water to cover to a boil in a large saucepan over high heat. Reduce heat to medium-low, and simmer until tender when pierced with a fork, about 15 minutes. Drain and cool 5 minutes. Cut potatoes in half.
Whisk together sour cream, mayonnaise, vinegar, garlic, oregano, chives, parsley, and lemon juice in a medium bowl. Gradually whisk in 1/4 cup of the olive oil, 1/4 teaspoon of the pepper, and remaining 1/2 teaspoon salt.
Toss potatoes with remaining 2 tablespoons olive oil and 1/4 teaspoon pepper in a large bowl. Grill potatoes, cut side down and covered with grill lid, until lightly charred, about 8 minutes. Turn and grill, covered with grill lid, 2 more minutes. Transfer to a bowl, and cool 5 minutes.
Toss together the cucumber, sweet onion, potatoes, and 1 cup of the dressing. Serve with remaining dressing.