How to Make It
Preheat the oven to 400°. In a large pot of boiling salted water, cook the beans until tender, about 4 minutes. Drain in a colander.
In a large nonstick skillet, heat the butter. Sauté the shallots over medium-high heat until golden brown and caramelized, about 8 minutes.
Sprinkle the flour over the shallots and cook for 1 minute, until incorporated. Whisk in the milk until smooth. Simmer until slightly thickened, about 5 minutes. Season with the nutmeg, 1 teaspoon salt and pepper. Add the beans and toss to coast.
Transfer the mixture to a 3-quart ceramic baking dish and sprinkle the breadcrumbs over the top. Bake for 15 minutes, until heated through and the top is lightly golden brown.