Preheat the oven to 425 degrees. In a large pot of salted boiling water, cook the cauliflower until barely tender, about 5 minutes; drain.
In a large skillet, melt the butter over medium heat. Add the flour and cook, stirring often, until the mixture is light golden, 1 to 2 minutes. While stirring constantly, add the milk, cheese and pumpkin spice. Stir until the cheese melts and the sauce thickens, about 2 minutes; season. Stir in the cauliflower. Transfer the mixture to a medium (2 qt.) baking dish. In a small bowl, mix the breadcrumbs and Parmesan; sprinkle over the cauliflower.
Bake until the casserole is golden brown and bubbly, 20 to 25 minutes.