Trim the ends off of the cucumber. Peel alternating strips for a visual pop if desired. Using a mandoline or sharp knife, cut the cucumbers into ⅛ inch slices. Transfer to a mixing bowl.
Slice the red onion into thin slices using the mandoline or sharp knife. Add to the bowl with the cucumbers. Set aside.
In a small bowl, combine the mayonnaise, sour cream, vinegar, sugar, salt, and pepper. Mix well.
Start by pouring ½ of the dressing over the cucumbers and onion. Gently toss to coat the vegetables with the dressing. Add extra dressing if desired. (Reserve extra dressing to use on other salads).
Store the cucumber salad in the refrigerator for up to 1 week.