Place potatoes in a colander and toss with lemon juice and salt. Let stand until potatoes are soft, about 10 minutes. Pat dry with a paper towel.
Spray a large baking sheet with nonstick cooking spray.
Tear off four squares of wax paper (about 12-inches square) and spray with nonstick cooking spray (preferably butter-flavored). Shingle all but four potato slices horizontally to form a 5x9-inch rectangle on each square. Spray potatoes with cooking spray.
Place salmon lengthwise over potatoes and spread with mustard.
Lift right side of wax paper and fold over salmon, covering it with potatoes. Lightly press paper to help potatoes adhere to fish and peel back wax paper. Repeat with left side of wax paper.
Lay remaining potato slices over seam where shingled potatoes meet and brush with butter.
With a spatula, carefully transfer salmon to baking sheet, turning bundles over so the side with the additional potato slice is down. Cover loosely with plastic wrap and refrigerate so potatoes set, about 30 minutes.
Preheat broiler. Spray potatoes with cooking spray. Broil 6 inches from heat until potatoes are golden and crisp, about 5 minutes on each side.