Green Bean Casserole with Crispy Shallot Rings

Serves 8 to 10
2 pounds green beans, trimmed and cut into 3-inch pieces
Pinch of kosher salt
6 tablespoons (3/4 stick) unsalted butter
1 onion, diced
1 pound button mushrooms, cleaned and sliced
1/4 cup all-purpose flour
2 1/2 cups milk
2 teaspoons sea salt
1/2 teaspoon freshly ground black pepper
2 grates of fresh nutmeg
Canola oil, for frying
9 shallots, sliced crosswise into 1/4-inch rings
1/2 cup cornstarch, sifted
Preheat the oven to 350°F. Lightly grease a 2-quart casserole dish.
Bring a large saucepan of water to a boil and add the kosher salt. Prepare a large bowl of ice water and set aside. Blanch the beans, cool and drain thoroughly.
Meanwhile, melt 2 tablespoons of the butter in a large skillet over medium heat, add the onion and cook and stir about 3 minutes, until soft and translucent. Add 1 more tablespoon of the butter, raise the heat to medium-high and sauté the mushrooms until golden-brown, about 4 minutes. Transfer to a large bowl and add the beans.
Melt the remaining 3 tablespoons butter in the same skillet over medium heat and whisk in the flour. Whisk until the flour turns light brown, about 2 minutes. Pour in the milk and continue whisking until the mixture thickens, about 4 minutes. Stir in the sea salt, pepper and nutmeg, remove from the heat and let cool completely. Pour the sauce over the bean mixture and stir to coat evenly.
Spoon the bean mixture into the prepared baking dish and place in the oven to bake about 30 minutes, until the sauce begins to bubble around the edges.
While the beans are baking, pour oil in a saucepan 3 to 4 inches deep and place over medium-high heat until sizzling hot.
Working in batches, toss the shallots with the cornstarch to coat, shaking off any excess, and carefully place in the oil to fry about 2 minutes, until golden-brown and crispy. Remove with a slotted spoon and place on a paper towel to drain.
Remove the casserole from the oven, sprinkle with the shallots and return to the oven to continue baking about 5 minutes more, until the casserole is heated through. Remove from the oven and serve warm.