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1
In a large saucepan melt butter over moderately low heat and whisk in flour until smooth.
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2
Cook roux, whisking, 3 minutes.
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3
Whisk in milk in a stream and salt and pepper to taste and simmer, whisking occasionally, 5 minutes, or until thick.
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4
Sauce may be prepared up to this point 3 days ahead and chilled, its surface covered with plastic wrap.
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5
Stir parsley and turmeric into sauce until combined well.
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6
In a very large heavy kettle heat oil over moderately high heat until hot but not smoking and saute onions, stirring, until they begin to turn golden.
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7
Reduce heat to moderate and cook onions, stirring occasionally, 30 minutes, or until most of liquid from onions is evaporated.
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8
Stir in sauce.
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9
Preheat oven to 375F.
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10
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.
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11
Spread about 1 cup sauce over pasta and sprinkle with about 3 tablespoons each of Fontina and Parmesan.
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12
Make 3 more layers in same manner, beginning and ending with pasta and making sure pasta is completely covered with sauce.
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13
Cover dish tightly with foil, tenting slightly to prevent foil from touching top layer, and bake in middle of oven 30 minutes.
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14
Remove foil and bake lasagna 10 minutes more, or until top is bubbling.
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15
(If desired, broil lasagna about 4 inches from heat 1 minute to brown top).
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16
Let lasagna stand 5 minutes before serving.
