Heat the oven to 350 degrees Fahrenheit.
If using fresh green beans, blanch the beans by bringing a large pot of water and 2 1/2 tablespoons of salt to a boil, then add green beans and boil for about three minutes. With a slotted spoon, remove green beans to a bowl of ice water. If using frozen green beans, thaw, rinse, and drain the green beans and set aside.
In a large skillet, melt the butter over medium-high heat. Add the mushrooms and cook for about 5–7 minutes or until reduced by half. Add garlic, one teaspoon of kosher salt, and 1/4 teaspoon pepper. Cook together for about one minute longer.
Sprinkle flour over the mushroom mixture and stir for about two minutes. Add broth and milk to the mixture. Continue stirring until the sauce thickens, about five minutes. Add the cream and cook for about two more minutes. Remove mushroom gravy from the heat.
Add the green beans and about eight ounces of the fried onions and gently mix until combined. Transfer the green bean mixture to a greased 9 x 13-inch baking dish. Sprinkle the remaining four ounces of fried onions and transfer to the oven for about 15 minutes or until the mixture is bubbly and the fried onions are crisp and brown. Allow to cool for about ten minutes and serve!