How to Make It
Preheat oven to 400ºF. Steam cauliflower over simmering water until crisp-tender, about 5 minutes. Transfer to a bowl.
In a large pan, warm 1 Tbsp. oil over medium heat. Add leeks and season with salt and pepper; sauté until tender, about 3 minutes. Add flour; cook, stirring, for 1 minute. Slowly stir in broth and half-and-half. Simmer, stirring, until thickened, about 4 minutes. Pour over cauliflower, add half of cheese and toss. Transfer to a wide, shallow 2-quart baking dish; cover tightly with foil and bake for 25 minutes.
In a bowl, mix bread crumbs, horseradish and remaining cheese. Sprinkle cauliflower with mixture, drizzle with remaining 1 Tbsp. oil and top with prosciutto. Bake until bread-crumb mixture is golden brown, 15 to 20 minutes.