8 ounces button mushrooms, diced to 1/4-inch pieces
2 small onions, 1 minced, 1 very thinly sliced
2 tablespoons butter
2 3/4 cups heavy cream
2 pounds haricots vert, chopped to 1-inch pieces
2 cups flour (there may be some left over)
Vegetable oil, as needed for frying
1 tablespoon chopped parsley
Salt and pepper to taste
10-by-10-inch shallow baking dish
Preheat the oven to 350 degrees. In a pan set over low heat, sweat the minced onion and the diced mushroom in the butter. When the onions are translucent, add the cream and reduce by half, stirring occasionally so that the mushrooms and onions don’t stick to the bottom of the pot. Remove from heat and set aside.
Bring a large pot of salted water to boil. Set up a large bowl of ice water. Blanch the haricots vert in boiling water, then shock them in the cold water. Drain and pat dry with a paper towel. Set aside. In a large pot, heat the vegetable oil to 250 degrees. Place the thinly sliced onions and flour in a large bowl and toss the onions until they are completely coated. Shake off the excess flour and fry the onions in the oil. When the onions are a light golden brown, remove them from the oil and drain them on paper towels.
In a bowl, combine the beans with the mushroom cream, then add 1/2 of the fried onions. Spoon the mixture evenly into the baking dish and bake for about 35 minutes. When ready to serve, pile the rest of the fried onions onto the center of the casserole and sprinkle with the chopped parsley.