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.
Coat skillet with cooking spray; place over medium-high heat until hot. Add shallots and 1 clove minced garlic; saute until tender. Stir in spinach and 1/8 teaspoon pepper.
Combine 1 tablespoon clam juice and cornstarch. Combine remaining clam juice, lemon juice, and 1/2 teaspoon garlic in a saucepan; bring to a boil. Reduce heat; simmer 1 minute. Stir in cornstarch mixture. Cook, stirring constantly, until mixture is thickened. Stir in remaining 1/8 teaspoon pepper. Combine breadcrumbs, cheese, melted margarine, and remaining 1 teaspoon garlic.
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; drizzle with sauce. Sprinkle with breadcrumb mixture. Broil 3 inches from heat (with electric oven door partially opened) 4 minutes.