Blanch the broccoli: bring a pot of salted water to a boil, add broccoli for 2 minutes, then plunge into ice water or cold water to stop cooking. Drain and set aside. This keeps them vibrant and crisp.
Melt the butter in a small saucepan over medium heat. Whisk in the flour to form a smooth paste (a roux) and cook for about 1 minute to lose the raw flour taste.
Slowly whisk in the milk, keeping the heat steady. Stir until the mixture thickens into a light, creamy sauce. If it looks lumpy, keep whisking or use a whisk to smooth it out.
Lower the heat and stir in the cheese, garlic powder, salt, pepper, and nutmeg if you’re using it. Let the cheese melt completely, stirring until you’ve got a glossy sauce. Don’t rush this step—silky is the goal.
Pour or spoon the cheese sauce over the broccoli, or toss to coat evenly in the pan. If you like it extra cheesy, add a last-minute dusting of cheddar on top and a quick broil for a melted cap.