Preheat oven to 350. Preheat a large (12-inch) nonstick T-fal skillet over medium until the Thermo-Spot turns solid red. Melt the butter in the preheated skillet and add the mushrooms, onion, and garlic. Cook, stirring until softened, about 6 minutes.
Add the flour to the skillet and cook, stirring constantly, 1 minute. While stirring, slowing pour in the chicken broth and half and half. Bring to a simmer over medium heat and cook, stirring, until the sauce thickens, about 7 minutes.
Remove the sauce from the heat and stir 2 cups of the cheddar cheese into the sauce until melted. Season with salt and pepper.
Stir the cooked pasta, green beans, and chicken into the cheese sauce. Top with remaining cup of cheese.
Transfer the skillet to the oven and bake until cheese is bubbly, about 5 minutes. Top with fried onions and serve.
Recipe by 5* trending recipes with videos at https://food.theffeed.com/2019/11/26/cheesy-green-bean-chicken-casserole/