Preheat oven to 350°. Cook green beans according to instructions on the bag; drain and sett aside.
In a 10-12 inch oven-proof skillet, over medium-high heat, melt the butter. Add the mushrooms, salt, pepper, onion powder, and garlic powder. Cook, stirring occasionally, until mushrooms are softened and give off some moisture, about 3 minutes.
Sprinkle the flour over the top. Cook, stirring constantly, for 3 minutes to let raw taste cook out of the flour. Whisk in the mushroom broth until smooth. Reduce heat to medium-low and whisk in half and half; cook until thickened, about 5-8 minutes.
Remove from heat and stir in 2/3 cup of the french fried onions and the green beans. Stir well to combine; top with remaining french fried onions. Bake 10-15 minutes until bubbly.
Substitute chicken or vegetable broth for the mushroom broth.