Preheat oven to 375°F (190°C). Butter a 9x13 inch baking dish.
Blanch the broccoli: Boil for 2–3 minutes until bright green and crisp-tender, then drain and set aside.
Prepare the sauce: In a saucepan, melt butter over medium heat. Whisk in flour and cook 1–2 minutes until lightly golden. Gradually whisk in milk, cooking until smooth and thickened. Stir in salt, pepper, and nutmeg.
Layer casserole: Spread half the broccoli in the baking dish, top with turkey, then cover with remaining broccoli. Pour the sauce evenly over the top.
Add cheese and breadcrumbs: Sprinkle shredded cheese over the sauce, then top with bread crumbs or crushed crackers.
Bake: 25–30 minutes, until bubbling and golden on top.
Rest: Let stand 5 minutes before serving to set the layers.