Ingredients

• Kosher salt

• 2 lb. 

  green beans, trimmed

• 12 tbsp. 

  (1 1/2 stick) unsalted butter, divided

• 2 

  yellow onions, thinly sliced

• 1 lb. 

  cremini mushrooms, sliced

• Freshly ground black pepper

• 4 

  cloves garlic, finely chopped

• 1/2 c. 

  all-purpose flour

• 6 c. 

  whole milk

• 1 1/2 c. 

  French's fried onions

Description

1. Step 1Preheat oven to 350°. Prepare an ice bath in a large bowl. In a large pot of generously salted boiling water, cook green beans until bright green, about 3 minutes. Using a slotted spoon or tongs, transfer beans to ice bath to cool. Drain and transfer to another large bowl.
2. Step 2In another large pot or saucepan over medium heat, melt 4 tablespoons 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, then transfer onion mixture to bowl with green beans.
3. Step 3In same pot over medium heat, melt remaining 8 tablespoons 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.
4. Step 4Bake green bean casserole until warmed through and bubbling, about 30 minutes.
5. Step 5Top with fried onions and continue to bake until warmed through, about 5 minutes more.