Trim and cut green beans. Then, add to a pot of boiling water and cook for five minutes. Drain and pour in ice water to stop the cooking process.
Melt butter and add the mushrooms, stir to coat and cook until reduced and browned, about 10 minutes. Stir occasionally. Remove from skillet and set aside.
Melt more butter and whisk in flour and garlic. Then, pour in the cream, chicken stock, seasonings and mushrooms. Cook until thickened and stir in Parmesan.
Mix in the green beans, water chestnuts and fried onion with the cream. Pour into a baking dish and top with more fried onions. Bake until hot and bubbly.