Preheat the oven to 400 degrees F.
Bring a pot of salted water to a boil. Once boiling, add in the green beans. Blanch for 5 minutes, then remove the green beans and place them in a bowl of ice water to stop the cooking.
Heat the olive oil and butter large skillet over medium-low heat. Add in the diced onion with a big pinch of salt and pepper. Cook until softened, about 5 minutes.
Stir in the mushrooms. Cook until they soften too, about another 5 minutes. Stir in the garlic and cook for 1 minute.
Stir in the flour to create a roux. Cook for 2 to 3 minutes, until it smells fragrant and a bit nutty. Slowly stream in the stock, stirring the entire time. Cook for about 5 minutes, until the mixture is thickened and simmering.
Stir in the cream until combined. Stir in the grated nutmeg and the parmesan cheese. Turn off the heat. Add the green beans to the cream mixture and toss to coat. Pour the mixture in a 9x13 baking dish. Sprinkle on extra parmesan cheese. Top with the french fried onions.
Bake the green beans for 20 to 25 minutes, until the sauce is bubbly and warm. Serve immediately!
To serve 4: Cut this recipe in half exactly. I suggest keeping the same amount of fried onions because they are delicious!
To serve 10 to 12: Double this recipe exactly. The same amount of onions works too - but you can also make more if you’d like.