How to Make It
In a large saucepan, melt the butter. Stir in the flour over moderate heat. Gradually whisk in the milk until smooth, then whisk in the cream and bring to a boil over moderately high heat, whisking constantly. Add the onion and bay leaves and simmer over low heat, whisking often, until the sauce is thickened and no floury taste remains, about 15 minutes. Discard the onion and bay leaves and add the shredded Fontina; season with salt and pepper.
Preheat the oven to 375°. Butter a 15-by-10-inch baking dish. In a large skillet, melt the 3 tablespoons of butter in the oil. Add the garlic and cook over moderately high heat until fragrant, about 1 minute. Add the chard, in batches if necessary, and cook over high heat, stirring, until the leaves are wilted and any liquid has evaporated, about 10 minutes. Season with salt and pepper and transfer to a colander to drain thoroughly, pressing down on the chard. Spread the chard in the prepared baking dish in an even layer and pour the béchamel sauce on top.
In a medium bowl, mix the bread crumbs with the 4 tablespoons of melted butter. Stir in the Parmesan. Spread the bread crumbs evenly over the gratin and bake for about 45 minutes, or until golden brown and bubbling. Let the gratin rest for about 10 minutes before serving.