Position a rack in the center of the oven and preheat to 350 degrees F. Grease a 9-by-13-inch baking dish with butter or cooking spray.
Whisk together the milk, half-and-half, cayenne, eggs, 3/4 teaspoon salt and 1/2 teaspoon pepper in a large bowl. Fold in the sausage, spinach, bagels and 3/4 cup of the provolone, then pour into the prepared baking dish. Dot the top with the cream cheese and sprinkle all over with the everything bagel seasoning and remaining provolone (see Cook's Note).
Cover with foil and bake for 45 minutes. Uncover and bake until the casserole is puffed, golden brown at the edges and set in the center, about 45 minutes more. Let cool 15 minutes before serving.
This casserole can be baked right away or assembled the night before, refrigerated and baked the following morning. Let the chilled casserole sit at room temperature for 15 to 20 minutes before baking.
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