Preparation

• Preheat oven to 350°. Prepare an ice bath in a large bowl. In a large pot of generously salted boiling water, cook green beans, stirring occasionally, until bright green, 2 to 3 minutes. Using a slotted spoon or tongs, transfer beans to ice bath to cool. Drain and transfer to another large bowl.
• In another large pot or saucepan over medium heat, melt 4 Tbsp. butter. Add onion and a large pinch of salt and cook, stirring occasionally, until starting to soften, about 7 minutes. Add mushrooms and cook, stirring often, until mushrooms are browned, about 7 minutes more. Add garlic and cook, stirring, until fragrant, about 1 minute; season with salt and pepper. Transfer onion mixture to bowl with green beans.
• In same pot over medium heat, melt remaining 8 Tbsp. butter. Add flour and cook, whisking frequently, until golden and smells toasty, about 3 minutes. Gradually whisk in milk; season with salt and pepper. Bring to a simmer and cook, stirring occasionally, until thickened, about 4 minutes. Add to bowl with green beans and stir to combine. Transfer to a 13" x 9" baking dish.
• Bake green bean casserole until warmed through and bubbling, about 30 minutes.
• Top with fried onions and continue to bake until warmed through, about 5 minutes more.