In a large skillet, saute onion in oil and 2 tablespoons butter until tender. Stir in broth and clam juice; bring to a boil. Add scallops, shrimp, crab and 1/8 teaspoon pepper; return to a boil. Reduce heat; simmer, uncovered, for 4-5 minutes or until shrimp turn pink and scallops are firm and opaque, stirring gently. Drain, reserving cooking liquid; set seafood mixture aside., In a large saucepan, melt the remaining butter; stir in flour until smooth. Combine milk and reserved cooking liquid; gradually add to the saucepan. Add salt and remaining pepper. Bring to a boil; cook and stir for 2 minutes or until thickened. , Remove from the heat; stir in cream and 1/4 cup cheese. Stir 3/4 cup white sauce into the seafood mixture., Preheat oven to 350°. Spread 1/2 cup white sauce in a greased 13x9-in. baking dish. Top with three noodles; spread with half of the seafood mixture and 1-1/4 cups sauce. Repeat layers. Top with remaining noodles, sauce and cheese. , Bake, uncovered, at for 35-40 minutes or until golden brown. Let stand for 15 minutes before cutting.