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1
Prepare parchment packets Heat the oven to 400F.
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2
Lightly season fish on both sides with salt and pepper.
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3
Cut four pieces of parchment paper (each measuring 12 by 17 inches) and lay them on a clean work surface.
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4
Fold each in half crosswise; then open, and lay flat.
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5
Divide spinach leaves among parchment, mounding it on one side of the fold.
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6
Divide the compound butter in half, then divide one of the halves among the four packets, spooning it on top of the spinach; top each with 2 lemon slices.
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7
Lay a fillet on top of the lemons and spoon remaining compound butter on fish.
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8
Fold over parchment and then form the half-moon packet: beginning at one of the corners, make small overlapping pleats all the way around to seal the edges completely.
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9
Steam packets Transfer packets to a rimmed baking sheet, and cook in the oven until parchment puffs up, about 12 minutes (8 minutes per inch of thickness for firm-fleshed fish).
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10
Serve Immediately place a packet on each of four plates and use kitchen shears to cut open the packets at the table.
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11
Serve with lemon wedges.
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12
Firm-fleshed fish varieties with plenty of flavor work best with the caper butter in this recipe; try halibut, salmon, or striped bass.
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13
Since they can be rinsed, salt-packed capers will actually taste less salty than those packed in brine and have a pure caper taste.
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14
Use nonpareil capers whenever possible.
