Preheat oven to 350ºF. Bring a large pot of water to a boil.
Separate the cauliflower into bite-sized florets. Add to the boiling water; cook for 5 minutes or until just tender. Drain.
While the cauliflower cooks, melt butter in a small saucepan over medium-high heat. Add the flour. Cook, stirring constantly, for 3 minutes or until light golden brown. Whisk in the milk, stock and mustard. Bring to a simmer and cook until the sauce thickens, about 5 minutes. Stir in the nutmeg. Slowly stir in half the gruyere.
Add the cauliflower, chard, and ham to a 9-inch casserole dish. Pour the sauce over the vegetables and top with remaining cheeses. Sprinkle breadcrumbs in a thin layer over the top.
Bake for 20 minutes, or until bubbly and golden brown. Broil for 5 minutes to help the breadcrumbs crisp. Let sit at least 5 minutes before serving.
Serves 6.