Oysters Rockefeller

Ingredients 1 pound fresh spinach Vegetable Cooking Spray 2 tablespoons minced shallots 1 clove garlic, minced 2 tablespoons butter, melted 1/4 teaspoon salt 1/4 teaspoon pepper, divided 3 tablespoons butter 3 tablespoons all-purpose flour 1 1/2 cups half-and-half 1 tablespoon lemon juice 36 oysters on the half shell 1 (4-pound) package rock salt 8 slices