Bring first 3 ingredients to a boil in a large saucepan over medium heat. Reduce heat, and add scallops; simmer 1 minute or until scallops are opaque and slightly firm. Strain scallop mixture, reserving liquid; set scallops and leeks aside. Return liquid to saucepan. Bring to a boil; cook, uncovered, 20 minutes or until liquid is reduced to 1 1/4 cups.
Combine flour and milk in a small heavy saucepan; stir until smooth. Cook over medium heat, stirring constantly, until thickened. Gradually add reduced liquid, stirring constantly. Cook, stirring constantly, until thickened. Add 1/4 cup cheese and next 4 ingredients; stir until cheese melts.
Combine scallop mixture and 3/4 cup cheese sauce; stir well. Spoon scallop mixture evenly into 6 individual gratin dishes coated with cooking spray. Spoon remaining 1 cup cheese sauce evenly over scallop mixture. Sprinkle evenly with remaining 1/4 cup cheese. Bake at 375° for 15 minutes or until bubbly.