Gather your ingredients. Preheat oven to 350ºF. Heat oil in a large skillet on medium-high heat.

Sautè the white mushrooms until the mushrooms are brown with lots of liquid in the pan, about seven minutes.  Do not drain the liquid.

Whisk together cornstarch, non-dairy milk, kosher salt, and black pepper in a small bowl until the cornstarch is mostly dissolved.

Pour the cornstarch mixture into your mushrooms, while the heat is still going. Stir constantly until thick and creamy, about three minutes.

Lightly spray a two-quart casserole dish with oil.  Add the green beans, then pour the mushroom sauce over them.  Stir lightly to coat. Make sure to smooth the surface of the casserole out evenly and top with crispy fried onions.

Bake for 40-50 minutes at 350ºF. 40 minutes yields tender-crisp green beans.  Keep the casserole in those 10 extra minutes if you prefer fully tender beans.

Serve hot! This casserole keeps beautifully in the fridge for five to six days or in the freezer. Reheat in the oven. Note, green beans will not stay tender-crisp after being frozen.