Cook onion and garlic in 2 teaspoons butter in a large nonstick skillet over medium- high heat 3 minutes or until tender, stirring constantly. Add clam juice and next 3 ingredients. Bring to a boil; cook 15 minutes or until reduced to 2 cups. Add remaining butter; stir until melted. Pour over cooked pasta. Add green onions and pepper to pasta mixture; stir well.
Place a large nonstick skillet coated with cooking spray over medium-high heat. Add scallops; cook, stirring often, 5 minutes or until done. Drain well. To serve, divide pasta evenly among 4 bowls; top with scallops.