INGREDIENTS
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4 tablespoons butter, plus more for baking dish
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2 cups uncooked elbow macaroni
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2 tablespoons minced onion
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4 tablespoons all-purpose flour
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1 teaspoon salt, plus more for seasoning
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3/4 tablespoon dry mustard
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1/4 teaspoon paprika
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2 cups milk
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3 cups shredded Cheddar, divided
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2 ripe tomatoes, blanched, peeled, and cut into 1/2-inch thick slices
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Freshly ground black pepper