Instructions

1. Bring a large pot of salted water to a boil over high heat. Meanwhile, prepare an ice bath by filling a large bowl halfway with ice and water. Arrange a rack in the middle of the oven and heat the oven to 350°F. Lightly coat a 9x13-inch or other 3-quart baking dish with unsalted butter.
2. Trim the stem ends from 1 1/2 pounds green beans. Thinly slice 5 large shallots (about 3 1/4 cups). Thinly slice 1 pound cremini or button mushrooms (about 5 cups). Thinly slice 3 garlic cloves. Juice 1 medium lemon until you have 1 tablespoon.
3. Add the green beans to the boiling water and cook until crisp-tender, about 4 minutes. Drain and transfer to the ice bath to stop the cooking. When cool, drain again and let sit in the colander while you make the sauce.
4. Melt 4 tablespoons unsalted butter in a large skillet over medium-high heat. Add 1/4 of the sliced shallots (about 2/3 cup), mushrooms, 1 1/4 teaspoons of the kosher salt, and 1/2 teaspoon black pepper. Cook until the moisture is released and the mushrooms begin to brown, 5 to 7 minutes. Add the garlic and cook until fragrant, about 1 minute.
5. Sprinkle 1/4 cup of the all-purpose flour evenly over the mushroom mixture and stir to combine. Cook until the flour is no longer dry and the film on the bottom of the pan turns golden, about 1 minute. While whisking constantly and scraping the bottom of the pan, slowly pour in 2 1/2 cups whole or 2% milk and 1/2 cup tahini.
6. Bring to a boil. Cook, whisking frequently, until the sauce thickens, 1 to 3 minutes. Remove the pan from the heat. Add the lemon juice and 1 tablespoon of the za’atar, and stir to combine. Taste and season with more kosher salt as needed.
7. Add the green beans and toss to coat in the sauce. Transfer the mixture to the baking dish and spread into an even layer. (Alternatively, leave the mixture in the skillet if it is ovenproof, spread it into an even layer, and bake directly in the skillet.) Bake until the sauce is bubbling and the green beans are tender, 15 to 20 minutes. Meanwhile, fry the shallots.
8. Line a plate with paper towels. Place the remaining shallots, 1 tablespoon all-purpose flour, and 1/2 teaspoon ground turmeric in a medium bowl and toss to coat. Heat 1 cup neutral oil in a medium saucepan over medium heat until shimmering. Add the shallots and fry, stirring frequently, until golden and crisp, 4 to 7 minutes. Using a slotted spoon, transfer the shallots to the plate and season with the remaining 1/4 teaspoon kosher salt and 1/2 teaspoon za’atar.
9. Sprinkle the green bean casserole with the fried shallots before serving.