Preheat the oven to 350 degrees and place a rack in the center of the oven. Butter an 8" square casserole dish and set aside.
Bring a pot of water to a boil and add the green beans to cook for 10 minutes. Drain the beans and run under cool water to stop the cooking process and set them aside.
Melt the butter in a large skillet over medium-high heat and add the onion. Cook for about 2−3 minutes then add the garlic and mushrooms. Season the mixture with 1 tsp kosher salt and continue cooking for an additional 6−8 minutes until the moisture from the mushrooms has evaporated and the edges become golden brown.
Sprinkle the mixture with the flour and pepper and stir to combine. Cook the flour for about 2 minutes before adding the chicken broth. After adding the broth, stir to dissolve the flour completely. Once the mixture comes to a boil, reduce the heat to medium-low and continue to simmer for about 2 minutes.
Stir in the half-and-half and simmer for about 5 minutes.
Add the green beans to the skillet and stir to coat thoroughly in the sauce before transferring to the casserole dish.
Bake the casserole for about 15 minutes until the edges begin to bubble and the casserole is hot throughout.
Top the casserole with the pureed fried onions and bake an additional 15 minutes until the topping becomes a nice deep golden color.
Remove the casserole from the oven and allow to rest for 5 minutes before serving. Enjoy!
One Last Thing
This recipe might be the one time you don’t send the leftovers home with your guests!