Slice each potato crosswise at 1/8 to 1/4-inch intervals. Cut almost to the bottom but not all the way through the potato. Slice down through the potato and when the knife hits the edge of the spoon, stop slicing.
TIP: For best results, choose oblong potatoes of uniform size and trim if needed so they'll lie flat without wobbling as you cut into them.
You can rest the potato in a large serving spoon and use that as a guide for when to stop slicing — slice straight down, and when your knife hits the edge of the spoon, stop slicing.