Parboil the potato slices in salted water for 5 minutes, then blanch broccoli florets for 2-3 minutes and drain well.
In a saucepan, melt the butter. Whisk in flour and cook until lightly golden. Slowly pour in the milk while whisking until smooth. Cook at low heat until thickened. Stir in the shredded cheese. Season with salt, pepper, and nutmeg if using.
Layer potatoes and broccoli in a baking dish. Season with salt, pepper, garlic powder and onion powder.
Pour the cheese sauce evenly over the vegetables. Grate a layer of Parmesan cheese on top.
Bake for 25–30 minutes, or until bubbling and golden on top.
Let cool for a few minutes, then serve warm.
Notes
Dry the broccoli well to prevent excess water in the bake.
For a crispier top, broil for the last 2–3 minutes.
Add cooked bacon or pancetta for a smoky variation.