Ingredients:
- To taste kosher salt
- 3 lbs. green beans, trimmed
- 1 1/2 sticks unsalted butter, divided
- 2 yellow onions, thinly sliced
- 1 lb. cremini mushrooms, sliced
- To taste freshly ground black pepper
- 4 cloves garlic, finely chopped
- 1/2 cup all-purpose flour
- 6 cups whole milk
- 1 1/2 cups fried onions
Directions:
- Preheat oven to 350 F. 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, 3 to 5 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 tablespoons of butter. Add onion and a large pinch of salt and cook, stirring occasionally, until starting to soften, about 8 minutes.
- Add mushrooms and cook, stirring often, until mushrooms are browned, about 6 minutes more.
- Add garlic and cook, stirring, until fragrant. Season with salt and pepper. Transfer onion mixture to bowl with green beans.
- In 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-by-9-inch 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.
