These are crisp potato wafers topped with smoked salmon and a dollop of sauce with dill and creme fraiche.
Make the sauce: Whisk together creme fraiche, lemon juice and zest, capers, shallot, dill, salt, and pepper in a small bowl. Refrigerate, covered, at least 1 hour (up to 6 hours). Makes about 3/4 cup sauce.
Make the chips: Using a mandoline fitted with a waffle blade or the ridged slicer on a box grater, cut each potato into 1/8-inch-thick slices, rotating a quarter turn after each slice to create a waffle pattern (discard end slices). Transfer to a large bowl of cold water while working to prevent discoloration. Potatoes can be refrigerated in water overnight.
Heat about 5 inches of oil in a medium, heavy stockpot until it registers 350 degrees on a deep-fry thermometer. Meanwhile, drain potatoes, and dry. Working in batches of about 8, fry potatoes, flipping once, until pale golden brown and crisp, about 1 minute 15 seconds per side. Using a slotted spoon, transfer to paper towels to drain. Season with salt. (Adjust heat between batches, as necessary, to keep oil at a steady temperature.) Let cool completely.
To assemble: Top each chip with a piece of salmon and about 1/2 teaspoon sauce. Garnish with lemon zest and a sprig of dill. Serve immediately.
Thin, even waffled potato slices are easiest to cut using a mandoline. You can substitute store-bought ridged potato chips for the homemade ones if you like.