Shuck oysters, reserving bottom shell;set aside. In a large skillet, sauté onion in butter until tender. Add spinach; cook and stir until wilted. Remove fromthe heat; stir in the cheese, lemon juice and pepper.
Spread sea salt into two ungreased 15x10x1-in. baking pans. Lightly press the oyster shells down into the salt. Place one oyster in each shell; top each with 2-1/2 teaspoons spinach mixture.
Bake, uncovered, at 450° until oysters are plump, 6-8 minutes. Serve immediately.