Instructions

1. Preheat the oven to 450 degrees F. Line a baking sheet with foil.
2. Toss the onions, flour, panko and salt in a large mixing bowl to combine. Spread the onions evenly on the baking tray, then drizzle with olive oil. Bake the onions until golden brown, approximately 30-35 minutes, rotating the sheet halfway through baking and tossing the onions a few times so that they don’t burn. Once they’re done, remove from the oven and set aside. Reduce the oven temperature to 400F.
3. While the onions are in the oven, make the beans: bring a gallon of water and 2 tablespoons of salt to a boil in a large pot. Add the beans and cook for 5 minutes, then drain and immediately run the beans under ice-cold water for 2 minutes, tossing frequently. (Or if you’re the kind of person with multiple counters, you could plunge the beans into a bowl of ice water. The point is to get them cold very quickly so that they stop cooking.) Drain and set aside.
4. Melt the butter in a 10- or 12-inch cast-iron skillet on medium-high heat. Add the mushrooms, 1 teaspoon of salt and pepper, and cook, stirring occasionally, until the mushrooms brown and begin to release their liquid, 5-7 minutes.
5. Stir in the garlic and nutmeg and cook for another 2-3 minutes. Stir in the flour and cook for 1 minute, then add the broth and simmer for 1 minute.
6. Reduce the heat to medium-low and stir in the cream. Cook until the mixture thickens, stirring occasionally, for 8-10 minutes.
7. Remove from the heat and stir in the green beans and a quarter of the onions. Scatter the remaining onions over top the mixture. Transfer to the oven and bake until the casserole is bubbling, about 15 minutes. Remove and serve immediately.