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1
Preheat the oven to 350u00b0 F with the rack in the middle. Butter a 2-quart shallow baking dish.
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2
Bring 1-inch salted water to a boil in a large saucepan. Meanwhile, wash lamb's-quarters in a large bowl of cold water and drain well. Trim any coarse, woody stems at bottom (don't go crazy with trimming; young lamb's-quarter stems cook up tender and delicious).
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3
Add lamb's-quarters to pot and cook over medium heat, covered, until leaves are wilted and stems are tender, 2 to 3 minutes. Drain the greens in a large sieve or colander and rinse well under cold running water. Drain again, pressing out excess liquid with the back of a large spoon. Coarsely chop greens and transfer to a bowl. Dry saucepan and reserve.
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4
Cook scallions in olive oil with 1/4 teaspoon salt in reserved saucepan over medium heat, stirring, until softened, 3 to 4 minutes. Combine scallion mixture with greens in bowl. Reserve saucepan again.
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5
Melt butter in reserved saucepan over medium-low heat and stir in flour. Cook, stirring, 2 minutes, then whisk in milk and bring to a boil, whisking. Reduce heat and simmer sauce, whisking, 2 minutes (it will be thick). Remove from heat and whisk in nutmeg, zest, and salt and pepper to taste.
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6
Mix sauce into greens mixture. If the mixture is too thick, thin with a little additional milk; season with salt and pepper. Spread out mixture in baking dish.
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7
Toss together crumbs, cheeses, and oil until combined well. Sprinkle topping evenly over greens mixture and bake in oven until crumbs are golden and mixture is bubbling, about 30 minutes.
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8
The best way to make coarse fresh breadcrumbs is to cube fresh bread, then grind it in small batches in a blender (a food processor is not as good at grinding crusty bread into breadcrumbs because the crusts get stuck in the blade).
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9
Creamy lamb's-quarters mixture and crumb mixture can be made 1 day ahead and kept separately in airtight containers, chilled.
