1. Slice lemon into 4 (1/16-inch) rounds. Cut rounds into 8 wedges.
2. Remove and discard skin from fish; flake fish.
3. Combine arugula, juice, and oil in a bowl; toss to coat.
4. Arrange apple slices in a single layer. Divide arugula mixture and fish evenly among stacks; top each with 1 lemon wedge.