Preheat the oven to 400 degrees F (200 degrees C).
Mix flour, bread crumbs, Parmigiano-Reggiano cheese, and five-spice powder together in a bowl. Cut in 1 1/2 tablespoons butter using your fingers until mixture resembles pebbles. Cover and refrigerate until needed.
Cut the stem off the broccoli and use a spiralizer to cut into ribbons. Cut the head into florets.
Melt 3 tablespoons butter in a large saucepan over medium heat. Add onion and garlic; saute until translucent, about 4 minutes. Add cornstarch, pepper, and nutmeg; mix well until roux begins to brown, 3 to 4 minutes. Add milk, stirring continuously, and continue to cook until sauce begins to thicken, 3 to 4 minutes more. Mix in 1/4 cup Cheddar cheese. Stir in broccoli until thickened, 2 to 3 minutes. Pour mixture into a baking dish.
Top mixture with remaining Cheddar cheese. Sprinkle crumble mixture evenly over the top. Reduce the oven temperature to 350 degrees F (175 degrees C).
Bake in the preheated oven until the crumble has browned and sauce is bubbling, about 20 minutes.