-In a large pot or Dutch oven, melt the butter on medium heat til melted.
-Add the mushrooms, salt and pepper and cook for about 10 minutes, stirring often, until the mushrooms are cooked.
-Add the garlic and thyme and cook for another minute or so.
-Sprinkle the flour over the mixture and stir to incorporate.
-A quarter cup at a time, stir in the chicken stock, letting addition become absorbed before adding another. The mixture will begin thickening as you go.
-Add the boullion cube and stir in the wine, cream and milk.
-Bring to a boil and let simmer over medium-low heat, stirring occasionally, until the sauce is thickened, about 15 minutes.
-Stir in the sherry. Taste the sauce and add salt and pepper, if needed. Since the beans won’t be seasoned, you want the sauce to be salty/peppery.
-While the sauce cooks, toss the beans and cornstarch well.
-Pour the mushroom sauce over the top and toss to combine.
-Transfer to a 9×13 ceramic baking dish.
-At this point, you can either freeze the dish, using the instructions below, or continue right on to baking.

To Freeze: Cool completely. Cover with plastic wrap, making sure there’s no air between the beans and the plastic. Wrap with foil. Freeze. Do not thaw before baking.

Topping: While casserole is baking, combine topping ingredients in a bowl and toss until the butter is evenly distributed. Set aside until the last 10 minutes of baking.

To Bake: Preheat oven to 400F.
*If casserole is frozen, remove plastic wrap from casserole, then replace the foil. Bake for 30 minutes, stir, leave the foil off and bake for another 50 minutes. Spread topping over the casserole, bake for another 10 minutes until golden.
*If casserole is freshly made or thawed, bake covered in foil for 30 minutes, stir and add the topping. Bake for another 10 minutes until the casserole is bubbly and the topping is golden.