Cauliflower Cheddar Gratin with Horseradish Crumbs

Preparation

1. Step 1

   Preheat oven to 450°F.

   Step 2

   Cook cauliflower in a 5- to 6-quart pot of boiling salted water until just tender, 6 to 8 minutes. Drain cauliflower well in a colander and transfer to a buttered 2-quart baking dish.

   Step 3

   While cauliflower is cooking, melt 2 tablespoons butter in a 3- to 4-quart 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, scallion greens, salt, and pepper, whisking until cheese is melted. Pour cheese sauce over cauliflower and stir gently to combine.

   Step 4

   Coarsely crumble crackers into a bowl. Melt remaining 2 tablespoons butter in a small saucepan, then remove from heat and stir in horseradish. Pour over crumbs and toss to coat.

   Step 5

   Sprinkle crumb topping evenly over cauliflower.

   Step 6

   Bake gratin in middle of oven until topping is golden brown, about 10 minutes.