Preheat the oven to 350°F (175°C).
Blanch the green beans in boiling salted water for about 3 minutes.
Drain the beans and immediately place them in an ice bath to stop the cooking process.
In a large skillet, heat olive oil over medium heat and sauté the onion until translucent, about 5 minutes.
Add the sliced mushrooms and garlic, cooking until the mushrooms are browned and the moisture has evaporated.
Pour in the heavy cream, seasoning with salt and pepper. Let the mixture simmer for about 5 minutes.
Combine the blanched green beans with the mushroom sauce in a large mixing bowl.
Transfer the mixture into a greased 2-quart baking dish and top with extra French fried onions.
Bake in the preheated oven for 25-30 minutes, or until bubbly and golden brown.
Let it cool for a few minutes before serving.