Swiss Chard Gratin with Toasted Bread Crumbs

Kosher salt
2 pounds Swiss chard (about 3 bunches), stems and large ribs removed and chopped, leaves cut into 1-inch ribbons
1 stick (4 ounces) unsalted butter
1 medium onion, finely chopped
4 medium garlic cloves, minced
1/4 cup plus 2 tablespoons all-purpose flour
1 teaspoon freshly grated nutmeg
3 cups half-and-half
2 tablespoons fresh lemon juice
Freshly ground pepper
6 slices country bread, ground into coarse crumbs in a food processor (about 3 cups)
1/4 cup extra-virgin olive oil
1/4 cup chopped flat-leaf parsley
2 teaspoons minced thyme

Preheat the oven to 350°. In a large pot of boiling salted water, cook the chard stems and ribs until almost tender, about 8 minutes. Add the chard leaves; cook until just wilted, about 2 minutes. Drain the chard, let cool slightly and squeeze dry. Working in batches, pulse the chard in a food processor until coarsely chopped. Transfer to a bowl.
Melt the butter in a medium saucepan. Add the onion and garlic and cook over moderate heat until softened, 6 minutes. Add the flour and nutmeg and cook, stirring, until smooth. Whisk in the half-and-half and bring to a boil, whisking until slightly thickened. Pour over the chard and mix well. Add the lemon juice; season with salt and pepper.
Transfer the creamed chard to a deep 8-by-12-inch baking dish and bake in the middle of the oven for 22 minutes, or until hot and bubbling.
Meanwhile, spread the bread crumbs on a rimmed baking sheet and toast on the bottom of the oven for 10 minutes, or until golden and crisp. Transfer to a bowl and toss with the olive oil, parsley and thyme. Season with salt and pepper.
Sprinkle the toasted bread crumbs over the gratin and serve warm.

Make Ahead The recipe can be prepared through Step 4 early in the day and refrigerated. Reheat, top with the bread crumbs and serve.