1 pound haricots vert, ends trimmed
1/2 pound shiitake mushrooms, very thinly sliced
1 tablespoon olive oil
2 tablespoons finely diced shallot
1/4 cup sherry
2 cups heavy cream
2 large onions
2 cups flour
Vegetable oil, for frying
Salt and pepper, to taste
Blanch the haricots vert in boiling water. Shock them in ice water and set aside. In a pan over high heat, sauté the shiitake mushrooms in olive oil. Add the shallots, then add the sherry. Turn the heat to medium and reduce until the mixture is almost dry. Add the heavy cream, turn the heat to low, and simmer until it is reduced by half. Toss in the haricots vert and transfer the mixture to a nine-inch rectangular casserole dish.
Use a very sharp knife to shred the onions into paper-thin slices. Toss them in the flour, seasoned with salt and pepper, and and deep-fry them in the oil until golden. Drain the fried onions on paper towels and pile them atop the green-bean casserole. Heat in the oven until heated throughout. Serve warm.