Step 1: Preheat your oven to 350°F (175°C). Blanch the green beans in boiling salted water for 3-5 minutes, then immediately shock them in an ice bath. Drain well.
Step 2: In a large skillet, melt the butter over medium heat. Add the sliced mushrooms and cook until browned and the moisture has evaporated.
Step 3: Stir in the garlic and cook for 1 minute until fragrant. Sprinkle the flour over the mushrooms and whisk for 1-2 minutes to cook the raw flour taste out.
Step 4: Gradually whisk in the broth and heavy cream. Simmer until the sauce thickens, about 5 minutes. Season with salt and pepper.
Step 5: Stir the blanched green beans and 1/2 cup of the fried onions into the mushroom sauce.
Step 6: Transfer the mixture to a 9x13 inch baking dish. Bake for 20 minutes.
Step 7: Top with the remaining 1 cup of fried onions and bake for an additional 5-10 minutes until the onions are golden brown and the sauce is bubbling.