Preheat oven to 350 degrees. In a skillet heat 2 tablespoons of butter. When hot add shallots, carrots and celery, cover and cook until wilted, about 5 minutes. Add the shrimp and crab and saute, stirring continuously for 3 to 4 minutes or until seafood is just cooked through. Stir in Cognac and evaporate over high heat. Stir in tomatoes and cook 10 minutes, uncovered, or until mixture is quite thick. Remove from heat and season to taste with salt and pepper.
Make a roux: Melt the remaining 4 tablespoons butter and add 4 tablespoons flour. Whisk in cream and milk and simmer five minutes or until thick. Remove from heat and stir in 1/2 cup of the gruyere cheese; season to taste with salt, pepper and nutmeg Cut the pasta sheets into strips 3 to 4 inches wide and long enough to fit into your gratin or baking dish. Boil, a few sheets at a time until 3/4 done. Drain and cool on clean cloths. Generously butter 3 quart capacity baking pan. Spread a thin film of cream sauce over bottom of baking dish. Arrange a layer of 4 overlapping sheets of lasagne. Spread a thin film of cream sauce over pasta, then seafood mixture. Top with pasta and repeat, reserving 1/3 cup of cream sauce for top layer. Finish with a layer of pasta, cream sauce and remaining 1/4 cup of gruyere. Bake, loosely covered with foil for 20 minutes. Uncover and bake 10 minutes or until hot throughout and only slightly golden on top. Let stand in pan for 5 minutes then serve.