makes one 9 x 13-inch baking pan
1/2 cup diced onion
2 Tbs butter
8 ounces thinly sliced mushrooms
2 pounds trimmed fresh green beans
water as needed for steaming beans
for the sauce:
4 Tbs butter, melted
1/4 cup all-purpose flour
2 1/2 cups milk
pinch cayenne pepper
salt and pepper to taste
grated parmesan cheese as needed (approx. 1/3 cup)
canned fried onion rings for garnish (optional)
Melt 2 Tbs of butter in a large saute pan and saute 1/2 cup finely diced onion to soften. At that point add 8 ounces thinly sliced mushrooms and cook those to brown up a bit.....and then turn the heat off.
While you're keeping your eye on the mushrooms, add about 1-inch of water to another large, wide, deep pan and add one 2-pound bag of trimmed fresh green beans....turn the heat to high and put a lid on top.
Once the water comes to a boil, steam the green beans for 4 - 5 minutes......and then very carefully drain them through a colander and run a little cold water over them to cool a bit.
Add them back to the pan, without any heat underneath, and add the cooked mushrooms as well.
Preheat oven to 350 degrees.
To make the sauce for the casserole, melt 4 Tbs butter in a deep, heavy bottomed stainless steel pot and then whisk in 1/4 cup all-purpose flour and cook that along for a minute.
Then add 2 1/2 cups milk and bring that up to a simmer....whisking it a little bit as you go.
Add a little pinch of cayenne pepper, a little pinch of nutmeg and a little salt and pepper to taste and, once it has thickened up nicely, turn the heat off.
At that point add the sauce to the mushrooms and green beans, toss to coat and carefully transfer it to a well butter 9 x 13-inch baking dish.
Sprinkle a little grated parmesan cheese over the top and pop it, uncovered, into a preheated 350 degree oven.
It'll take about 30 minutes in the oven and then you can sprinkle some optional store bought fried onions over the top before serving.
Assemble ahead of time and keep wrapped and cold in the fridge until time to bake....just give it an extra 20 minutes or so to warm completely through.