Step 1

Bring 1 quart of water to a boil in a medium pot. Add the spinach and cook until very tender and the water is green, 5 to 6 minutes. Drain the spinach in a colander set over a large bowl and reserve 2 3/4 cups of the cooking liquid. Let the spinach sit until cool enough to handle, then finely chop, and set aside.

Step 2

Melt the butter in a medium pot over moderately high heat. When the butter is foamy, add the onions, celery and garlic and cook, stirring, until softened, about 3 minutes. Add the reserved spinach water, bring to a boil and cook for 1 minute. Add the chopped spinach, liqueur, salt and pepper and simmer over medium heat, stirring occasionally, until the mixture reduces slightly, about 10 minutes. Remove from heat, add the cracker meal, and stir well to combine. Cool completely before using.

Step 3

Preheat the oven to 400°F. Spread a 1/2-inch-thick layer of rock salt on a large baking sheet and across the bottoms of 6 large plates.

Step 4

Arrange reserved oyster shells on top of the salt on the baking sheet. Put 1 oyster in each shell and top with 2 to 3 tablespoons of the sauce, spreading the sauce evenly out to the edge of the shell to completely cover the oyster. (Alternatively, transfer the sauce to a pastry bag fitted with a plain tip and pipe the sauce over the oysters.)

Step 5

Bake until the sauce is lightly browned and the oysters begin to curl around the edges, 20-25 minutes.

Step 6

Using tongs or a spatula, carefully transfer the hot shells to the salt-covered plates and serve immediately.