Preheat oven to 350°F (175°C).
In a large pot, bring water to a boil and cook the green beans for 5 minutes, then drain and set aside.
In a skillet over medium heat, melt the butter and sauté the garlic and mushrooms until tender.
Add the heavy cream, cheddar cheese, salt, and pepper to the skillet. Stir until the cheese is melted and the sauce is smooth.
Combine the green beans with the cheese sauce and half of the French fried onions in a casserole dish.
Bake in the preheated oven for 20 minutes.
Sprinkle the remaining French fried onions on top and bake for an additional 5 minutes until golden brown.
Serve hot and enjoy!