Bring a large pot of salted water to a boil over high heat. Blanch the cauliflower for 2 minutes. Transfer the cauliflower to a buttered 9 by 13-inch baking dish and let cool, about 5 minutes.
In a small skillet over medium-high heat, brown the pancetta. Transfer to a small bowl and set aside.
In the same skillet, melt the butter. Add the flour and stir until combined. Continue stirring for 2 minutes to cook the flour. Slowly add the milk, stirring continuously until the milk has incorporated and there are no lumps. Add the garlic clove and red pepper flakes and stir the sauce over low heat until the mixture coats the back of a wooden spoon. Add the Parmesan, salt, pepper, nutmeg, and the reserved pancetta. Stir to combine. Pour the sauce over the cauliflower to cover. Top with a sprinkling of bread crumbs.
Bake until the breadcrumbs are golden and the cauliflower is cooked, about 25 minutes. Serve immediately.