Preheat oven to 350℉ degrees and grease a 9x13 inch baking dish.
In a large microwave safe bowl, add the green beans and 3/4 cup water. Microwave on high for 5-8 minutes or until just tender. Drain the beans well and set aside.
In a large skillet, melt the butter over medium-high heat. Add the mushrooms, salt, and pepper and cook until the mushrooms have released their juice. Add the garlic, and cook for an additional 30 seconds.
Sprinkle the flour on top and cook for a minute. The mixture will be thick. Whisk in the chicken broth and bring to a simmer. Stir in the cream and simmer for 8-10 minutes or until mixture thickens.
Pour sauce over green beans, 1/2 cup of the fried onions and toss well to combine. Pour into the prepared pan. If making this ahead of time, cover tightly and store in the fridge.
Bake for 20-25 minutes or until green beans are to desired doneness (I like mine on the more cooked side). For the last 5 minutes of cooking, sprinkle the remaining fried onions on top and cook until golden brown.
Notes
Frozen beans: Fresh is best, but thawed, well-drained frozen beans work; skip the microwave step.
Make ahead: Make the sauced bean mixture, spread in the dish, cover tightly and refrigerate up to 24 hours. Bake from cold for about 10 extra minutes, then add the onions for the last 5 minutes so they stay crisp.
Leftovers: Refrigerate covered up to 3–4 days. Reheat at 350°F until hot; the onions soften.