Creme fraiche is in the dairy section of many grocery stores. It adds a tangy note to the dressing, but more sour cream can be substituted.
Arrange lettuce on a serving platter. Top with radishes, scallions, and cucumber. Refrigerate 10 minutes.
Whisk sour cream, creme fraiche, vinegar, and honey in a small bowl. Drizzle dressing over salad. Serve immediately