We bring extra umami (a savoriness) to a traditional green bean casserole with the addition of dried shiitake mushrooms. Once rehydrated, they turn soft and earthy, and their soaking liquid adds oomph to the sauce as well. Our fried onion topping is made from scratch – first, a dip in buttermilk, then fried to crunchy perfection.
Make the casserole: Place the dried shiitake mushrooms in a small bowl and cover with boiling water, about 2 cups, and cover with plastic wrap. Let steep for 20 minutes. Strain through a fine mesh strainer, reserve 1/2 cup of the liquid and slice the steeped mushrooms.
Melt the butter in a large Dutch oven over medium heat. Add the diced onion; cook, stirring, until softened, 7 to 10 minutes. Add the cremini and shiitake mushrooms; saute until softened and most of the liquid has evaporated, 10 minutes. Sprinkle in the flour; cook, stirring, 1 minute. Add the broth and mushroom liquid. Bring to a boil, then reduce the heat to medium-low and simmer until thickened, 5 to 10 minutes.
Bring a pot of salted water to a boil. Add the green beans and cook until crisp-tender, 7 to 10 minutes. Drain; shake off any excess water and add to the mushroom sauce. Cook, stirring occasionally, until the beans are very tender and the sauce is thick, 10 to 12 minutes. Stir in the heavy cream and season with 2 teaspoons of salt and pepper.
For the topping: Soak the sliced onions in the buttermilk, 5 minutes. Combine the flour and 1 teaspoon salt in a medium bowl. Heat 2 to 3 inches vegetable oil in a large pot until a deep-fry thermometer registers 350 degrees F. Working in batches, drain the onion, toss with the flour and shake off the excess, then fry until golden, 4 to 6 minutes. Drain on paper towels.
Transfer the green bean mixture to a 4-quart baking dish and top with the fried onions. (The casserole and topping can be made up to 2 hours ahead; refrigerate the casserole. Bring to room temperature; reheat at 350 degrees F, 30 minutes. Top with the fried onion; bake 5 more minutes.)
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