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

While cauliflower cooks, melt 2 tbsp. butter in a 3-4 quart heavy saucepan over moderately low heat and whisk in flour. Cook, 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.

Coarsely crumble crackers into a bowl. Melt remaining 2 tbsp. 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 and bake until topping is golden brown, about 10 minutes.