Blanch the Green Beans:
Bring a pot of salted water to a boil. Add green beans and cook 4–5 minutes until bright green and tender-crisp. Drain and rinse under cold water.

Make the Mushroom Sauce:
In a large skillet, melt butter over medium heat. Add onions and cook 3–4 minutes until softened. Add garlic and mushrooms and cook until mushrooms release their moisture, about 5 minutes.

Thicken the Sauce:
Sprinkle flour over the mushrooms and stir to coat. Cook 1 minute. Slowly add broth while stirring, then add heavy cream. Season with salt, pepper, and onion powder. Cook until thickened, 3–4 minutes.

Combine:
Remove from heat. Add the blanched green beans and 1 cup crispy fried onions. Stir to combine.

Bake:
Transfer mixture to a greased baking dish. Bake at 375°F for 20 minutes. Add extra fried onions on top and bake for an additional 5 minutes until golden.