Preheat the oven to 400 degrees
Add the butter, mushrooms, garlic, onion, salt, and pepper to a skillet
Cook the mushroom mixture over medium heat for 10-15 minutes, or until the mushrooms have cooked down and cooked off their liquid
Drain the french style green beans
In a bowl, stir together the drained green beans, cooked mushrooms and onions, and cream of mushroom soup
Grease a 9 by 13 baking dish and transfer the green bean casserole to it
Top the green bean casserole with 1 cup of grated sharp cheddar cheese
Bake the green bean casserole at 400 degrees for 20 minutes, or until bubbly
Remove the casserole from the oven and top it with the french fried onions. Bake it for another 5 minutes, or until the french fried onions are golden brown and crunchy on top.