Preheat your oven to 350 degrees F. Spray a 2 quart baking dish with cooking spray. Set aside.
Melt butter in a large skillet over medium heat. Add onions and cook for 2 minutes, stirring often. Add mushrooms and cook until softened, stirring often (about 3 to 4 minutes). Add garlic and cook for 30 seconds, while stirring.
Add soup and heavy cream. Stir to combine. Bring to a simmer, then add cheese, salt and pepper. Stir until cheese is melted.
Add green beans and ½ cup of the French fried onions. Gently stir to coat as evenly as possible.
Transfer to prepared baking dish and spread evenly.
Bake (uncovered) for 25 minutes. Using oven mitts, carefully remove from oven and sprinkle evenly with the remaining 1 ½ cups of the French fried onions. Return to oven and bake for an additional 5 minutes, or until onions are golden brown.
Let stand for 10 minutes before serving (sauce will thicken as it cools).