Cauliflower Cheddar Gratin with Horseradish Crumbs
3 pounds cauliflower (1 large head)
4 tablespoons unsalted butter
2 tablespoons flour
1 1/2 cups milk
6 ounces cheddar cheese (2 cups), grated
1/2 cup scallion greens, finely chopped
1/2 teaspoon salt
1/2 teaspoon pepper
20 2 inch saltine crackers
2 tablespoons drained bottled horseradish
Preheat oven to 450 degrees. Cook cauliflower in a 5-6 quart pot of boiling salted water until just tender (6-8 minutes). Drain well in a colander and transfer to a buttered 2 quart baking dish.
While cauliflower is cooking, melt 2 tablespoons butter in a 3-4 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, whisking occasionally. 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.
Coarsely crumble craskers 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. Sprinkle crumb topping evenly over cauliflower. Bake gratin in middle of oven until topping is golden brown, about 10 minutes.