03In 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.
04Bake green bean casserole until warmed through and bubbling, about 30 minutes.
05Top with fried onions and continue to bake until warmed through, about 5 minutes more.