INGREDIENTS
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Kosher salt
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8 ounces medium pasta shells
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3 tablespoons unsalted butter
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1 small onion, chopped
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1 tablespoon tomato paste
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1 tablespoon chili powder
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8 ounces ground beef
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2 tablespoons all-purpose flour
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1 cup low-fat (1 percent) milk
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2 1/2 cups shredded Mexican cheese blend (about 8 ounces)
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2 tablespoons breadcrumbs (preferably panko)
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2 scallions, thinly sliced