Preheat the oven to 375. Butter a large baking dish and set aside.
Next, do yourself a favor and prepare your mise en place. Basically this is a fancy French term for “everything in place.” As in, dice the onion and bell pepper and chop up the mushrooms so everything is ready to go.
Next, melt two tablespoons of butter in a heavy skillet over medium heat. Sautee the onions until they are softened and translucent, about 3-4 minutes.
Toss in the bell pepper and mushrooms and stir everything together.Let the veggies cook until most of the liquid has evaporated, about 8-10 minutes.
Next, cook the green beans according to the instructions on the package. (I prefer the stovetop method, but if you want to microwave them, no judgment here).
Once they are cooked, add the green beans to a bowl of ice water. This will prevent them from continuing to cook and prevents mushiness.While the green beans are cooling off, grate the cheeses. Sample accordingly. Mmmmm. Gruyere.
Drain the green beans and add them to the rest of the veggies, mixing to combine. Add a little salt and pepper to taste. Set aside.
Add the remaining 4 tablespoons of butter to a large saucepan over medium heat. Want to save yourself from doing more dishes? Use the same saucepan used to cook the green beans.
Once the butter has melted, add ¼ cup flour and whisk constantly until the mixture begins to turn golden brown. Congratulations – you just made a roux. (Hint: a roux is also the key to delicious turkey gravy – it’s essentially a thickening agent).
Add the milk in batches and continue to whisk. This part requires a little muscle and a lot of patience, but it will result in a delicious, creamy sauce. Also known as a béchamel. This is the foundation for most creamy sauces (and the basis for the most decadent macaroni and cheese you will ever indulge in… but that’s a recipe for a different day).
Add a little salt, pepper, cayenne pepper and nutmeg. Then stir in most of the cheeses, reserving a little for later. Taste. Swoon.
Pour the cheese sauce into the veggie mixture and toss to combine thoroughly.
Add the veggie mixture to the baking dish. Top with the remaining cheeses.
Now it’s time for the finishing touch – the crispy fried onions. I love the Trader Joe’s brand, but you really can’t go wrong here.
Cover with foil and bake for 20 minutes at 375. Remove the foil and continue to bake for an additional 10-20 minutes until browned and bubbly.
You can also make the green bean casserole up to one day ahead. Just be sure to bring it to room temperature before baking
Serve. Enjoy. Fight your loved ones for the last portion. It’s THAT good.