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1
Preheat oven to 450 degrees F. Coat the inside of a 9 by 13-inch glass or ceramic baking or gratin dish with 1 tablespoon butter.
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2
Cut 2 thin slices from each lemon half and squeeze juice from remaining lemon.
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3
Mash together remaining 2 tablespoons butter, 1 teaspoon lemon juice, and shallot and season well with salt and pepper.
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4
Sprinkle dish with remaining juice.
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5
Cook lettuce in a large pot of salted boiling water for 1 minute.
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6
Immediately transfer with tongs to a bowl of ice water to cool.
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7
Drain lettuce and cut out ribs, leaving top 1-inch of leaves intact.
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8
Spread out 1 lettuce leaf lengthwise on a work surface.
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9
Place another leaf on first leaf, overlapping enough to cover any holes (use 2 extra leaves if necessary).
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10
Season 1 piece halibut with salt and pepper and place crosswise in center of lettuce.
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11
Spread top of fish with 1/4 of shallot butter.
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12
Wrap lettuce over fish to form a packet (don't worry if ends aren't covered by lettuce) and transfer to baking dish.
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13
Make 3 more packets in same manner, arranging them close together in baking dish.
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14
Top each packet with a lemon slice.
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15
Cover packets with a sheet of waxed or parchment paper, then tightly cover dish with foil.
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16
Bake in middle of oven until centers of packets are just firm to the touch, 15 to 20 minutes, depending on thickness of fish.
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17
Serve with pan juices poured over packets.
