1. Preheat oven to 400 degrees F. Place a rimmed baking sheet in oven to heat. Cut parchment to size.
2. Stir together butter, capers, parsley, and garlic in a small bowl; set aside.
3. Divide potato slices between parchment rectangles, layering them in stacks to form a bed (slightly larger than the salmon fillet) to one side of crease. Season with salt and pepper.
4. Top each bed of potatoes with one quarter of the shallots followed by half the spinach. Place salmon fillets on top of spinach. Divide remaining shallots evenly between packets. Top each salmon fillet with 2 lemon slices (reserve remaining slices for garnish). Dot with caper butter; season with salt and pepper.
5. Fold parchment paper over ingredients, at the crease. Starting with one end of the paper and keeping edges together, make small overlapping pleats the length of the paper, creasing tightly as you go and shaping the edge into an arc. The packet should resemble a half-moon.
6. Carefully transfer packets to preheated baking sheet. Bake until packets have puffed, 15 to 18 minutes. Transfer packets to plates. Serve immediately, opening packets at the table. Garnish with remaining lemon slices.

Yield: 2 servings