Preheat the oven to 350 degrees F.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, 1/3 of the French fried onions. Add salt and pepper to taste.Pour the mixture into a 1 1/2 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.