Prepare lasagna noodles according to package directions: drain. Add tarragon to white sauce. In greased 13x9-inch baking dish, layer 1/3 noodles, 1/2 seafood, 1/3 each mozzarella, parmesan, and white sauce; repeat. Top with remaining noodles, white sauce, and cheeses. Bake in preheated 350 degrees F oven for 30 minutes or until bubbly. Let stand 10 minutes before serving.
*To prepare white sauce, melt 6 Tbsp butter or margarine in saucepan over low heat. Stir in 6 Tbsp flour; cook until well blended. Gradually add 3 cups milk, stirring constantly until smooth and thickened.