Prepare the Crispy Onions: In a large bowl, toss the sliced onions with flour, salt, and pepper. Heat about 1 inch of vegetable oil in a large, heavy-bottomed pot or deep fryer to 350°F (175°C). Fry the onions in batches until golden brown and crispy, about 2-3 minutes per batch. Remove with a slotted spoon and drain on paper towels.

Blanch the Green Beans: Bring a large pot of salted water to a boil. Add the green beans and cook for 3-4 minutes, until bright green and slightly tender-crisp. Drain immediately and transfer to an ice bath to stop the cooking process. Drain well. Toss with olive oil, salt, and pepper.

Make the Mushroom Sauce: In a large skillet or Dutch oven, melt the butter over medium heat. Add the sliced mushrooms and shallots and cook until softened and lightly browned, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the mushroom mixture and cook for 1-2 minutes, stirring constantly, to create a roux. Gradually whisk in the chicken broth, making sure to scrape up any browned bits from the bottom of the pan. Bring to a simmer and cook until the sauce has thickened slightly, about 5-7 minutes.

Stir in the heavy cream, sherry (if using), and nutmeg. Season with salt and pepper to taste. Reduce heat to low and simmer for another 5 minutes, allowing the flavors to meld.

Assemble the Casserole: Preheat oven to 350°F (175°C). In a large bowl, combine the blanched green beans and the creamy mushroom sauce. Gently toss to coat. Pour the mixture into a 9x13 inch baking dish.

Top evenly with the crispy fried onions. Bake for 20-25 minutes, or until the casserole is bubbly and the onions are golden brown. Let cool slightly before serving.