Prepare the Green Beans: Bring a large pot of salted water to a boil. Add the green beans and cook for 3-4 minutes, until crisp-tender. Drain and immediately plunge into an ice bath to stop the cooking process. Drain well and set aside.

Make the Homemade Mushroom Sauce: In a large saucepan, melt the butter over medium heat. Add the onion and cook until softened, about 5 minutes. Add the mushrooms and garlic and cook until the mushrooms are tender and browned, about 8 minutes.

Stir in the flour and cook for 1 minute, stirring constantly. 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 minutes.

Stir in the heavy cream, thyme, and sherry (if using). Season with salt and pepper to taste. Remove from heat.

Prepare the Crispy Fried Onions (if making homemade): In a large bowl, combine the flour, salt, and pepper. Toss the onion slices in the flour mixture until well coated.

Heat about 2 inches of vegetable oil in a large, heavy-bottomed pot or Dutch oven to 350°F (175°C). Working in batches, carefully add the onion slices to the hot oil and fry until golden brown and crispy, about 2-3 minutes per batch. Remove with a slotted spoon and drain on paper towels.

Assemble the Casserole: Preheat oven to 350°F (175°C). In a large bowl, combine the blanched green beans and the mushroom sauce. Toss to coat evenly.

Pour the green bean mixture into a 9x13 inch baking dish. Top with the crispy fried onions.

Bake for 20-25 minutes, or until the casserole is heated through and the onions are golden brown. Let stand for 5-10 minutes before serving.