2poundsgreen beanstrimmed and halved into smaller pieces
3tablespoonsbutter
8ouncescremini mushroomsquartered
4clovesgarlicminced
2tablespoonall-purpose flour
2cupsheavy cream
1 1/2teaspoonsItalian seasoning
1/2teaspoonkosher salt
1/2teaspoonblack pepper
1/4teaspoonground nutmeg
1/4cupfresh Parmesan cheesegrated
Instructions
Preheat the oven to 400 degrees F. Line a large baking sheet with parchment paper and set aside.
Combine onions and buttermilk in a medium bowl. Let soak for 15 minutes, then drain and discard the buttermilk.
Return the drained onions to the bowl. Sprinkle them with flour, season with salt and pepper, and toss to coat.
Transfer the onions to the prepared baking sheet. Coat them generously with cooking oil spray.
Bake for 25-30 minutes, tossing once or twice, until tender and crisp. Set aside to cool slightly. Reduce the oven temperature to 375 degrees F.
Mushroom Sauce
While the onions are baking, make the casserole. Bring a large pot of water to boil. Cook green beans for 7-9minutes, until tender. Immediately drain the beans into a colander and set aside.
In a skillet set over medium heat, melt the butter.
Add the mushrooms and cook for 6-8 minutes, stirring occasionally, until golden brown and reduced in size. Add the garlic and cook for another 2 minutes.
Sprinkle the flour over the mushrooms, stir, and cook for 2 minutes. Stir the heavy cream, Italian seasoning, salt, pepper, and nutmeg into the skillet and whisk well to combine. Continue cooking for about 5 minutes, until thickened. Remove from the heat, taste, and season with more salt and pepper, if desired.
Transfer the green beans to a large bowl and toss with the Parmesan. Pour the mushroom sauce over the beans and toss to coat.
Transfer the beans to the casserole dish and evenly distribute the onion topping over the top. Bake for 20-25 minutes, until the onions are golden and the sauce is bubbly.
Notes
Storing: Any leftover casserole can be stored in an airtight container in the fridge for 3-4 days. Freezing: The best way to freeze fresh green bean casserole is uncooked and without the onion topping. Prepare the mushroom sauce and the green beans and place in a casserole dish. Tightly wrap the dish with plastic wrap, and then a layer of aluminum foil. Freeze for up to 6 months. Reheating: To cook the frozen uncooked casserole, preheat the oven to 350F and cook for 45-60 minutes or until the casserole is warm through the center. Add the onion topping and bake for an additional 15 minutes or until the onion top is golden brown.