How to Make It
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.