Onion, Parsley, and Fontina Lasagna

In a large saucepan melt butter over moderately low heat and whisk in flour until smooth. Cook roux, whisking, 3 minutes. Whisk in milk in a stream and salt and pepper to taste and simmer, whisking occasionally, 5 minutes, or until thick. Sauce may be prepared up to this point 3 days ahead and chilled, its surface covered with plastic wrap. Stir parsley and turmeric into sauce until combined well. In a very large heavy kettle heat oil over moderately high heat until hot but not smoking and sauté onions, stirring, until they begin to turn golden. Reduce heat to moderate and cook onions, stirring occasionally, 30 minutes, or until most of liquid from onions is evaporated. Stir in sauce. Preheat oven to 375 °F. Pour 1 cup sauce into a flameproof baking dish, 13 by 9 by 2 inches (sauce will not cover bottom completely), and cover with 3 lasagna sheets, making sure they do not touch each other. Spread about 1 cup sauce over pasta and sprinkle with about 3 tablespoons each of Fontina and Parmesan. Make 3 more layers in same manner, beginning and ending with pasta and making sure pasta is completely covered with sauce. Cover dish tightly with foil, tenting slightly to prevent foil from touching top layer, and bake in middle of oven 30 minutes. Remove foil and bake lasagna 10 minutes more, or until top is bubbling. (If desired, broil lasagna about 4 inches from heat 1 minute to brown top). Let lasagna stand 5 minutes before serving.