Bring salted water to a boil. Add green beans and cook for five minutes. While the green beans cook, fill a bowl with ice and add water. When the green beans finish cooking after five minutes, drain and immediately add the green beans to the ice water to stop the cooking process.
In a skillet over medium heat, melt ¼ cup of the butter. Add the sliced mushrooms, stir to coat and cook until reduced and browned, about 10 minutes. Stir occasionally. Remove from skillet and set aside.
Add remaining butter to the skillet and melt. Whisk in the flour. Continuously whisk for about three minutes. Add garlic and cook for about 30 seconds.
Slowly pour in stock and heavy cream. Whisk until the mixture smooths out. Stir in soy sauce, pepper, Worcestershire sauce and ground nutmeg and add mushrooms back to the sauce. Simmer for about 10 minutes, whisking occasionally.
Add the Parmesan, stirring until it melts into the sauce. Turn off the heat and pour into an airtight container. In a separate airtight container, store the green beans with water chestnuts and keep both in the refrigerator for up to two days. This is helpful is refrigerator space is an issue. Alternatively, you can go ahead and pour the mushroom sauce and green beans into a 9-inch-by-13-inch baking dish, cover and store in the refrigerator.
Before baking in a 350-degree oven, let the mushroom sauce and green beans sit in room temperature for an hour or two. Pour the mushroom sauce into a 9-inch-by-13-inch baking dish and stir in the green beans, water chestnuts and 1 cup Crispy Fried Onions.
Place in 350-degree oven uncovered and bake until bubbly on the edges. Since this won't be warm off the stove, it will take longer to get to the bubbly stage. Start checking it after about 25 minutes, but it could take longer if the mixture is on the colder side. Once bubbly, top with remaining Crispy Fried Onions and bake for another five minutes. Take out of the oven and allow it to set for about five minutes and serve. Enjoy!