Step 1

Bring a large pot of water to a boil. Fill a large bowl halfway with ice water. Set a colander in the ice water.

Step 2

When the water comes to a boil, add 2 tablespoons salt and the green beans. Cook until the beans yield slightly when pierced with the tip of a knife but are not cooked through, about 4 minutes. Using a strainer or a slotted spoon or tongs, remove the green beans from the hot water and dump them into the colander in the ice water. Swirl the beans around so they cool quickly. Lift the colander out of the ice bath, let the beans drain, and set aside.

Step 3

In a 10-inch cast-iron or another ovenproof skillet, melt the butter over medium heat. Add the mushrooms, season with salt, up to 1 teaspoon cayenne, if using, and the mustard. (If kids will be partaking, you may wish to tone down the cayenne.) Stir to blend and cook until the mushrooms give off most of their liquid, 3 to 5 minutes.

Step 4

Using a whisk, add the garlic and 2 tablespoons of flour. When the flour has been incorporated, add the chicken broth and bring to a boil. Taste for seasoning and adjust accordingly. Stir in the cream and sour cream and gently simmer over low heat until the mixture thickens, 3 to 5 minutes. You should have about 2 cups. Turn off the heat and set the skillet aside.

Step 5

Pour the oil into a frying pan and heat it over medium heat until it registers 350ºF (176ºC) on a deep-fry or candy thermometer. Line a baking sheet with a kitchen towel and set out a slotted spoon.

Step 6

In a medium bowl, combine the remaining 1/4 cup flour and cayenne to taste, if using. Toss the onion rounds in the flour mixture and shake off any excess by shaking the rounds in a strainer. Test the oil by dropping in a single onion slice. It should begin to bubble and fry gradually.

Step 7

Drop a small batch of onions into the oil and gently swirl them as they fry. When they are light to medium brown, which ought to take 1 to 2 minutes, remove them with the slotted spoon and lay them out on the kitchen towel to cool. Sprinkle with salt. Repeat until all of the onions have been fried.

Step 8

Preheat the oven to 350ºF (176ºC).

Step 9

To assemble the green bean casserole, stir the green beans into the skillet containing the mushroom mixture. Simmer over low heat just until the green beans become tender when pierced with the tip of a knife, 10 to 15 minutes.

Step 10

Stir in half the onions. Slide the skillet into the oven for 10 minutes to give the green bean casserole a baked effect. Top with the remaining onions and serve immediately.