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1
Bring a large saucepan of salted water to boil. Thinly slice the reserved chard stems, add them to the water, and cook for 2 minutes. Add the leaves and continue cooking until tender, about 3-4 minutes longer. Drain and allow to cool, then gently squeeze out the excess liquid and coarsely chop.
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2
In the meantime, preheat the oven to 350 degrees. Melt 1 tablespoons of butter and toss the breadcrumbs, then spread them out on a small baking sheet. Toast, stirring occasionally, until lightly browned, about 10 minutes.
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3
In a medium saucepan, melt 3 tablespoons of butter over medium heat, then add the onion. Sweat until translucent, 5-7 minutes, then stir in the chard and a couple pinches of salt. Cook for 3 minutes, then sprinkle the flour over the chard and stir well to coat. Add the milk and nutmeg and bring to a boil, then simmer for 5 minutes as the mixture thickens. The chard should be just moist, but not overly wet, or else the gratin won't brown properly. Taste and season with salt.
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4
Butter a baking dish and spread the chard mixture evenly in the dish. Dot with the remaining butter and top with the breadcrumbs. Bake at 350 degrees until golden and bubbling, 20-30 minutes. Allow to cool for a few minutes, then serve.
