Remove and discard stems from spinach. Wash leaves; pat dry. Coat a nonstick skillet with cooking spray; place over medium-high heat until hot. Add one-third of spinach; saute 1 minute or until wilted. Set aside. Repeat procedure twice. Chop spinach.
Saute shallots and garlic in 2 tablespoons butter over medium-high heat until tender. Stir in spinach, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
Melt 3 tablespoons butter in a heavy saucepan over low heat. Add flour; stir until smooth. Cook 1 minute, stirring constantly. Add half-and-half; cook over medium heat, stirring constantly, until mixture is thickened. Stir in lemon juice and remaining 1/8 teaspoon pepper. Stir in spinach mixture.
Scrape a knife between oysters and shells to free meat; set shells aside. Place oysters in a colander to drain.
Sprinkle rock salt in two 15- x 10- x 1-inch jellyroll pans; arrange reserved shells on salt. Place 1 oyster in each half shell. Top with spinach mixture; sprinkle with bacon and cheese. Broil 3 inches from heat (with electric oven door partially opened) 4 minutes.