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Macaroni & Cheese Squash Casserole

MAKES 8 TO 12 SERVINGS

This cheesy mashup of squash casserole and mac and cheese might become a new covered-dish staple.

6 tablespoons unsalted butter, divided
2 medium yellow squash, halved lengthwise and cut into ½-inch-thick slices (about 4 cups)
1½ cups chopped onion
2 tablespoons all-purpose flour
1½ cups whole milk
1 tablespoon hot sauce
1 teaspoon kosher salt
½ teaspoon dry mustard
¼ teaspoon