We talk about reminiscence and recreating family recipes all the time when cooking and the green bean casserole is, for sure, a Thanksgiving classic. Oddly, this is a dish my mother NEVER made when I was growing up. I think I tasted it for the first time when I was about 30 years old. I know the classic base uses store-bought soup mix and fried onions. If you can cut the onions thinly using a mandoline slicer, that makes the task easier. Of course, store-bought fried onions can also be a good shortcut if you want to save time. It is also surprising how rewarding it can be to make it all from scratch!
Cook the green beans: In a large pot, bring 6 quarts of water to a boil. Salt the water generously; it should taste like sea water.
Prepare an ice bath: Fill a large bowl halfway with ice and add some cold water. Place a colander squarely inside the ice bath; the colander will keep you from having to pick the beans out from amongst the ice cubes. Plunge the green beans into the water and cook 2 to 3 minutes. When the beans are cooked, remove them from the hot water with a strainer and transfer them to the colander inside the ice bath. Use a spoon to "swirl" the beans around in the ice bath so they cool quickly. Drain.
Make the mushroom base: In a 10-inch cast-iron skillet, melt the butter and add the mushrooms. Season with salt and add the mustard and cayenne. Cook until the mushrooms give off most of their liquid, 5 to 8 minutes. Whisk the garlic and flour into the mushrooms. Cook the flour 1 to 2 minutes, stirring constantly so it doesn't stick or burn, and then add the chicken stock. Bring to a simmer and reduce 1 to 2 minutes. Stir in the cream and sour cream and simmer gently. Continue cooking over low heat until the mixture thickens, 5 to 8 minutes. Set aside.
Preheat the oven to 350 degrees F.
Fry the onions: Pour the oil into a medium frying pan. Heat to 350 degrees F.
In a medium bowl, combine the flour and cayenne. Toss the onion rounds in the flour and "shake" off excess by shaking the rounds in a strainer once they have been breaded. Test the oil by dropping in a few slices; they should rise to the surface and fry gradually.
Prepare a baking sheet fitted with a kitchen towel. Drop a small batch of the onions into the oil and gently "swirl" with a slotted spoon in the oil as they fry; this will help them fry more evenly on all sides. Fry until golden brown, 1 to 2 minutes. Remove from the oil and spread on the baking sheet. Season immediately with salt.
Bake and serve: When ready to serve, stir the green beans into the mushrooms. Simmer on the stove over low heat for 2 to 3 minutes so the beans and sauce meld together. Add more cream and a splash water if the sauce is too thick. Transfer the skillet to the oven for 8 to 10 minutes to finish heating it and to give the casserole that "baked" effect. Remove from the oven and top with the fried onions. Serve immediately.