How to Make It
Preheat the oven to 400°.
In a large nonstick skillet, heat the butter. Sauté the onion over medium-high heat until soft, about 5 minutes. Add the chard and cook until wilted, about 3 minutes. Add the spinach and cook 2 minutes more. In a small bowl, whisk together the milk and cornstarch. Add the pan and simmer until slightly thickened, about 5 minutes. Season with the nutmeg, 1 teaspoon salt and pepper.
Transfer the greens to a 3-quart ceramic baking dish and sprinkle the cheese over the top, followed by the breadcrumbs. Bake for 15 minutes, until heated through and the top is lightly golden brown.