Preheat oven to 450 degrees.
Cook cauliflower in a pot of boiling salted water until just tender, 6 to 8 minutes. Drain cauliflower well in a colander and transfer to a 1-quart baking dish coated with non-stick cooking spray.
While cauliflower is cooking, melt 1 tablespoon butter in a heavy saucepan over moderately low heat and whisk in flour. Cook roux over low heat, whisking, 3 minutes. Add milk in a slow stream, whisking, and bring to a boil, whisking frequently. Reduce heat and simmer sauce, whisking occasionally, 8 minutes. Remove from heat and add cheese, salt and pepper, whisking until cheese is melted. Pour cheese sauce over cauliflower and stir gently to combine.
Add breadcrumbs to a small bowl. Melt remaining tablespoon butter; pour over breadcrumbs and toss to coat. Sprinkle crumb topping evenly over cauliflower.
Bake gratin in middle of oven until topping is golden brown, about 10 minutes.
Bake, Stovetop
2005, 2006