Wash and peel the potatoes, then cut into thin slices and leave to sit in saltwater for 10min.
Preheat the over to 180-200 degrees celsius.
Wash the broccoli and cut into florets. Add to boiling salt water and cook for 8 min, until semi-soft. Remove the broccoli from the pot, rinse with cold water, drain and put to the side.
In the meantime, dice the onion and cut the ham into squares and briefly sweat in a pan with some oil. Keeping the heat low, add a small heaped tablespoon of flour to the pan, followed by the cream and milk. Mix well. While it cooks, add the salt, pepper and nutmeg. If the sauce thickens too much, add a dash of the broccoli water (or some vegetable broth, or plain water) to turn the sauce a bit runnier.
Lightly oil the base of a baking dish, then add the potatoes and broccoli to it. Pour the onion-ham sauce into the dish, then generously sprinkle the grated over the top.