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1
Preheat oven to 500F.
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2
Bring a 6- to 8-quart pot three-fourths full of water to a boil with kosher salt, then stir in spinach and cook 30 seconds.
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3
Transfer with a slotted spoon to a large bowl filled with ice and cold water to stop cooking.
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4
Drain spinach and squeeze dry, then finely chop.
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5
Wash leeks well in a bowl of cold water, then lift out and drain well.
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6
Heat oil in a 10-inch heavy skillet over moderately high heat until hot but not smoking, then saute bacon, stirring, until golden and just cooked through (but not crisp), about 3 minutes.
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7
Transfer with slotted spoon to paper towels to drain.
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8
Add leeks, shallot, and garlic to fat in skillet and cook over low heat, covered, stirring occasionally, until softened, about 8 minutes.
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9
Toss together leek mixture, spinach, bacon, bread crumbs, herbs, lemon zest and juice, sea salt, white pepper, nutmeg, and butter in a large bowl with a fork.
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10
Spread 2 to 3 cups rock salt in each of 2 large shallow baking pans and nestle oysters (in shells) in it.
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11
Spoon a heaping tablespoon bread-crumb stuffing loosely on top of each oyster.
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12
Bake in batches in upper third of oven until golden, about 6 minutes.
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13
Serve warm oysters in shells on plates lined with rock salt.
