3 cups rock salt
12 large oysters, well scrubbed (preferably Atlantic or Pacific oysters)
2 tablespoons unsalted butter
2 large shallots, minced (about 1/4 cup)
¼ teaspoon salt
⅛ teaspoon ground black pepper
Pinch hot red pepper flakes
2 medium garlic cloves, minced
4 ounces baby spinach (5 cups loosely packed)
1 slice high-quality white sandwich bread, such as Pepperidge Farm, crusts removed, torn into 4 pieces
¼ cup loosely packed fresh parsley leaves
2 tablespoons Pernod
2 tablespoons grated Gruyére cheese
1 lemon, cut into wedges
1. Adjust one oven rack so that it is 6 inches from the broiler element. Spread 1/2 inch of rock salt into two 9-inch pie plates or cake pans or other appropriate dish (see note). Shuck the oysters, reserving the oyster liquor in a small bowl. Gently nestle 12 half-shells holding one oyster each into the rock salt.
2. Heat the butter in a large nonstick skillet over medium heat until the foaming subsides. Add the shallots, salt, pepper, and red pepper flakes and sauté until the shallots soften, about 2 minutes. Add the garlic and sauté until fragrant, about 30 seconds. Add the spinach and sauté, tossing occasionally, until the spinach has fully wilted, about 1 1/2 minutes. Remove the pan from the heat and set aside.
3. Pulse the bread in the workbowl of a food processor fitted with a steel blade until it turns to coarse crumbs (resembling Grape-Nuts), about ten 1-second pulses. Add the reserved spinach mixture, the reserved oyster liquor, parsley, Pernod, and Gruyère. Pulse, scraping the sides of the workbowl down as necessary, until the mixture forms a smooth paste, about ten 1-second pulses. Spoon 1 tablespoon of the spinach mixture on top of each oyster.
4. Broil the oysters until spotty brown, about 10 minutes. Allow the oysters to cool for several minutes before serving them straight from the hot, salted dish with lemon wedges.