Preheat the oven to 350°. Butter a 9-by-13-inch glass or ceramic baking dish. In a large pot of boiling salted water, cook the noodles until al dente; drain and rinse well under cool water to stop the cooking. Drain again. Spread the noodles in the baking dish.
Meanwhile, in a medium skillet, melt 3 tablespoons of the butter over moderate heat. Stir in the crumbs just until evenly moistened. Remove from the heat.
In a large saucepan, melt the remaining 4 tablespoons of butter over moderate heat. Whisk in the flour and cook, whisking constantly, for 2 minutes. Remove from the heat. Whisk in the milk. Return the pan to moderate heat and bring the white sauce to a simmer, whisking constantly. Add the bay leaf and onion, reduce the heat to low and cook, stirring, until thickened, about 10 minutes. Season with salt and pepper.
Strain the sauce over the noodles. Stir in the Gruyère until thoroughly combined. Sprinkle the bread crumbs over the noodles. Bake for 35 minutes, or until bubbling and golden. Serve at once.
The baked gratin can be refrigerated for up to 2 days.
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