Trim cauliflower and divide into florets. In a large pot of boiling salted water, cook florets until just tender, about 6 minutes. Drain and run under cold water to stop the cooking.

Melt 1/2 tablespoon butter in a small saucepan over moderate heat. Add bread crumbs and stir to coat with butter. Season with salt. Cook, stirring, until bread crumbs are golden brown, about 5 minutes. Set aside.

Melt 2 tablespoons butter in a small saucepan over moderate heat. Add flour and whisk to blend. Cook 1 minute, then add milk, broth, bay leaf and garlic. Bring to a boil, whisking, then adjust heat to maintain a bare simmer and cook 8 to 10 minutes, scraping sides of pan with a spatula occasionally. Season with salt and pepper. Remove bay leaf and garlic. Keep warm.

Preheat oven to 400°F. Cut florets in half or quarters and arrange in buttered low-sided oval baking dish just large enough to hold them in one layer. Season with salt and pepper. Pour sauce over the cauliflower. Tear prosciutto into shreds and scatter over sauce. Top with cheese, then with bread crumbs. Bake in lower third of oven until bubbly and browned, about 30 minutes. Let rest 15 minutes before serving. Serve with chicken, lamb or pork.

To make fine bread crumbs, use two slices white sandwich bread, crusts removed. Toast crumbs in a preheated 375°F oven, stirring occasionally, until lightly colored and dry, 8 to 10 minutes. Cool, then process again until fine.