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1
Cook onions in oil in a 12-inch heavy skillet over medium heat, stirring frequently, until softened, about 7 minutes. Add garlic and cook, stirring, 1 minute. Increase heat to high, then stir in lamb and cook, stirring occasionally and breaking up any lumps with a fork, until meat starts to brown, about 15 minutes.
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2
Meanwhile, drain both cans of tomatoes, reserving juice, and chop.
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3
Pour off excess fat from skillet, then stir in tomatoes with their juice, thyme, spices, and 1 1/2 teaspoons salt. Simmer over low heat, partially covered, until some liquid has evaporated but sauce is still moist, about 40 minutes. Discard thyme.
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4
Melt butter in a heavy medium pot over medium heat. Whisk in flour, then cook roux, whisking frequently, until pale golden, about 6 minutes.
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5
Meanwhile, heat milk in a heavy medium saucepan until just about to boil. Add milk to roux in a stream, whisking constantly until very smooth. Bring sauce to a boil over medium heat, whisking, then cook, whisking, 1 minute. Remove from heat and stir in nutmeg, cheese, and 1 tablespoon salt.
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6
Lightly beat yolks in a bowl. Gradually whisk in 2 cups bechamel, then whisk yolk mixture into remaining bechamel in saucepan. Cover with a round of buttered wax paper (buttered side down).
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7
Preheat oven to 375u00b0F with rack in middle. Cook ziti in a pasta pot of boiling salted water (3 tablespoons salt for 6 quarts water) until al dente.
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8
Drain pasta, then transfer to a large bowl and stir in 1 cup bechamel.
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9
Arrange one third of pasta lengthwise in 1 layer in a large (17-by 11-inch) baking pan (about 2 inches deep).
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10
Add half of meat sauce (about 3 cups) to baking pan, spreading evenly over pasta.
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11
Make another layer of pasta and top with remaining meat sauce, then arrange remaining pasta on top.
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12
Spread remaining bechamel (about 6 cups) evenly over top layer of pasta.
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13
Stir together bread crumbs and cheese and sprinkle evenly over top of pastitsio.
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14
Bake until crumbs are golden brown and sauce is bubbling, about 45 minutes.
