Carefully shuck the oysters, reserving their liquor and bottom shell, and set aside. In a small saucepan over medium heat, combine the heavy cream and lemon zest, then reduce until the cream has thickened, about 10 minutes. Reduce heat and whisk in the butter, one piece at a time, until fully incorporated. Whisk in the lemon juice, then season with salt and pepper.
Add oysters and their liquor to the sauce, then cook over a very low heat, until the edges of the oysters begin to curl. Using about 1 tablespoon spinach (depending on the size of the shell), make a small “bed” in the oyster shell, lay one oyster in the bed, and spoon a small amount of sauce over the top. Serve immediately.