Heat the butter in a skillet over medium-high heat. Cook the onions until soft and translucent, 5-7 minutes. Add the mushrooms and garlic and cook until the mushrooms have released their juices and are tender, 5 minutes.
Stir in the flour and cook for another minute. Add the milk, stirring constantly to prevent lumping, and bring to a simmer. Continue stirring until the mixture is thickened. Add the chicken bouillon, salt and pepper. Cover, reduce the heat to medium, and simmer for another 5 minutes, stirring occasionally.
Stir in the green beans and, if desired, ⅓ of the French fried onions. Add salt and pepper to taste.
Pour the mixture into a 1½ quart casserole dish. Bake for 30 minutes until hot. Sprinkle with the remaining French fried onions and bake for another 5 minutes until the onions are golden brown.
Serve immediately.
Recipe by The Daring Gourmet at https://www.daringgourmet.com/easy-green-bean-casserole-from-scratch/