Preheat an oven to 350 degrees. Fill a large mixing bowl with ice cubes and water leaving about 3 inches from the top to allow the green beans to rest in the bowl.
Next, bring a large pot of water to a boil, add green beans and cook until bright green. Using a slotted spoon or tongs, quickly transfer the green beans to the ice bath. Once cooled, drain and transfer the green beans to a large bowl.
In a large 10 or 12 inch ovenproof skillet, melt 2 tbsp of butter over medium heat. Add the onion and cook until tender, about 5 minutes. Next, add in the mushrooms, kosher salt, and pepper. Continue to cook until softened, about 5 minutes more. Next, stir in the garlic and cook until fragrant. Transfer the mixture to the bowl with the green beans.
In the same skillet, melt the remaining 2 tbsp butter and whisk in the flour. Cook until golden, about 2 minutes. Add the homemade turkey stock and simmer for 3 minutes. Decrease the heat to medium-low and add the half-n-half, Dijon mustard, and Worcestershire sauce. Stirring occasionally, cook until the mixture is thick-about 10 minutes or maybe longer if you prefer a thicker sauce.
Next, add the green bean mixture to the skillet with the sauce and toss until combined. Bake until warmed through and bubbling, about 30 minutes. Top with fried onions and bake for 5 more minutes or until golden brown.