Best Homemade Green Bean Casserole

I find recipe ideas all over the place.  This one comes from Delish and it doesn't disappoint.

1 lb green beans, trimmed

6 Tbsp butter, divided

1 onion, sliced into half moons

8 oz sliced mushrooms

2 cloves garlic, minced

1/4 cup flour

3 cups whole milk

Kosher salt

Freshly ground black pepper

1 1/2 cups French's fried onions

Preheat your oven to 350 degrees.  Prepare an ice bath.  In a large pot of boiling water, add green beans and cook until bright green, about 6 minutes.  With a skittle spoon or tongs, quickly transfer green beans to ice bath to cool, then drain and transfer to a large bowl.  

In a large cast iron skillet over medium heat, melt 2 Tbsp of butter.  Add onions and cook, stirring occasionally until tender, about 5 minutes.  Add mushrooms and season with salt and pepper.  Cook, stirring often, until mushrooms are golden, about 5 minutes more.  Stir in garlic then transfer mixture to the bowl with the green beans.

Melt remaining 4 Tbsp of butter in the same skillet over medium heat.  Whisk in flour and cook until golden, about 2 minutes.  Gradually whisk in milk and bring to a simmer.  Cook until thickened, about 4 minutes.  Remove from heat then add green bean mixture and toss until evenly combined.  

Bake until warmed through and bubbling around the edges, about 20 minutes.

Top with fried onions and bake 5 - 8 minutes more.  Serve warm.