Green bean casserole is a classic holiday side dish to pair with that Thanksgiving turkey or Christmas ham. This green bean casserole is made entirely from scratch with a creamy mushroom sauce and crispy fried onions.
In a bowl, pour buttermilk over sliced onions and toss to coat. Set aside. Combine flour, panko, salt, and pepper in a second bowl.
Begin heating a saucepan with 1-2 inches of oil over medium-high heat on the stove top.
A handful at a time, transfer onions from buttermilk into panko mixture (allow excess buttermilk to drip off onions first). Toss quickly to coat, then transfer into the hot oil. Fry for about 2-3 minutes, or until lightly browned and crispy. Transfer to a paper towel lined plate. Continue until all onions have been fried.
Preheat oven to 375 degrees. Grease a 1.5 quart dish or 8x8 baking pan and set aside.
Meanwhile, bring a gallon of salted water to a boil. Prepare an ice bath (water and ice) in a large bowl and set aside. Add green beans to boiling water and cook for 5 minutes. Drain beans and immediately transfer to the ice bath to stop the cooking process. Once cooled, drain beans thoroughly and set aside.
In a sauté pan over medium heat, add butter. Once melted, add diced mushrooms and cook 3-4 minutes, or until lightly browned. Add minced garlic and cook for 1 minute. Add flour, stir to coat mushrooms, and cook for 1 minute. Add chicken broth and half and half, whisking to incorporate. Decrease heat to medium-low. Add dried thyme, salt and pepper. Cook, stirring occasionally, until mixture thickly coats the back of a spoon, about 4-5 minutes.
Remove from heat and add green beans to mushroom sauce. Stir to coat, then pour into prepared dish. Sprinkle top of casserole with fried onions.
Bake for 15-18 minutes, or until mixture is hot and bubbly. Remove from oven and serve immediately.
To make ahead: Prepare casserole as directed, but before adding onions or baking, cover and refrigerate overnight, storing onions separately. When ready to bake, preheat oven to 375 degrees, add onions to the top of your casserole, and bake for 18-22 minutes, or until bubbly and warmed through.
If doubling, serve in a 9x13 or 3 quart casserole dish.
Leftovers will keep for up to 4 days in a tightly sealed container in the refrigerator.