Position a rack in the lower third of the oven and preheat to 500 degrees ; or preheat the broiler. In a large pot, bring a few inches of water to a boil, salt it, add the cauliflower and cook until crisp-tender, about 5 minutes. Drain and return to the pot.
Meanwhile, in a saucepan, melt the butter over medium heat. Add the shallot and cook for 3 minutes. Sprinkle in the flour and stir for 1 minute. Whisk in the wine, then the milk; heat through and season with sea salt, pepper and nutmeg to taste. Cook until thick enough to coat the back of a spoon, 2 to 3 minutes. Stir in the mustard and gruyere until melted, then remove from the heat.
Stir the ham and the gruyere sauce into the cauliflower, then transfer to a large, shallow casserole. Top with the parmigiano-reggiano and bake or broil until brown and bubbly, about 5 minutes. Remove from the oven and top with the parsley.