In a bowl, pour buttermilk over 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, and transfer into hot oil. Fry for 2-3 minutes or until lightly browned and crispy. Transfer to paper towels. Continue until all onions have been fried.
Green Beans and Mushroom Sauce
Preheat oven to 375 degrees. Grease a 1.5 quart dish or 8x8 baking pan and set aside.
Bring a gallon of salted water to a boil over high heat. Meanwhile, 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 place into ice bath to stop cooking. Once cooled, drain beans 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 and stir to coat mushrooms. 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 thickens, 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.
MAKE AHEAD TIPS: Prepare casserole as directed without baking or adding onions to the top. Cover unbaked casserole 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.
To double this dish, serve in a 9x13 or 3 quart casserole dish.