Preheat the oven to 375°F (190°C). (Baking temperature remains the same)
Bring a very large pot of water (or two large pots) to a boil and blanch the green beans for 5 minutes. If your pot is not large enough to comfortably blanch all 3 pounds of green beans at once, blanch them in two batches. Transfer them to an ice bath to stop the cooking process. You may need a larger ice bath or two separate ice baths to cool the doubled amount of beans quickly. Drain and set aside.
In a skillet (or larger skillet/Dutch oven), cook the chopped bacon until crispy. You might need to cook the bacon in batches if your skillet is too small to avoid overcrowding. Remove and set aside.
In the same skillet (or larger skillet/Dutch oven), melt 4 tablespoons butter. Add the diced onion and cook until softened. If you are using a standard skillet and it seems too crowded with double the onions, you may want to cook the onions in two batches, using 2 tablespoons of butter for each batch. Stir in the minced garlic and cook for another minute.
Add the 2 cans of cream of mushroom soup, 1 cup heavy cream, and 1 cup shredded marble cheese to the skillet. Stir until the mixture is smooth and the cheese has melted. Season with salt and pepper to taste. Start with the recommended doubled amounts (1 tsp salt, 1/2 tsp pepper) and adjust to your preference.
In an extra-large bowl, combine the blanched green beans and the cream mixture. Mix until evenly coated. Ensure the larger quantity of beans is thoroughly coated in the sauce.
Spread the green bean mixture into a greased 10x15-inch baking dish (or two 9x13-inch dishes). If using two 9x13 inch dishes, divide the mixture evenly between them.
Top the casserole(s) with the cooked bacon, 1 1/2 cups shaved Parmesan cheese, and 1 cup crispy onion toppers (if using). Distribute the toppings evenly if using two baking dishes.
Bake in the preheated oven for 25-30 minutes, or until the top is golden and bubbly and the casserole is heated through. Due to the increased volume, it might take a few extra minutes to bake compared to the original recipe. Check for doneness around 25 minutes and add more time if needed. If using two baking dishes, they might bake slightly faster than one large dish, so check for doneness at 25 minutes.