Prepare the Brussels Sprouts: Bring a large pot of heavily salted water to a rolling boil. Add the trimmed and halved Brussels sprouts and blanch for 3-4 minutes, until they are bright green and slightly tender. Drain them immediately and plunge them into a large bowl of ice water to stop the cooking process. Once cooled, drain them thoroughly and pat them dry with a clean kitchen towel or paper towels. This step is crucial for ensuring a non-watery casserole.

Cook the Bacon: In a large, deep skillet or Dutch oven, cook the chopped bacon over medium heat until it is brown and crispy, about 8-10 minutes. Use a slotted spoon to transfer the crispy bacon to a paper towel-lined plate. Do not discard the bacon grease! Pour all but 2 tablespoons of the rendered fat into a small heatproof bowl and set it aside.

Sauté the Aromatics and Sprouts: Return the skillet with the 2 tablespoons of bacon grease to medium heat. Add the chopped shallots and cook until softened, about 3-4 minutes. Add the minced garlic and cook for another minute until fragrant. Add the blanched and dried Brussels sprouts to the skillet. Sauté for 5-7 minutes, stirring occasionally, until they begin to brown and caramelize in spots. Transfer the sprout mixture to a large mixing bowl.

Make the Cheese Sauce: In the same skillet (no need to wipe it out), melt the 4 tablespoons of butter over medium heat. Whisk in the flour and cook for 1-2 minutes, stirring constantly, to form a blond roux. Slowly pour in the warmed milk and heavy cream, whisking constantly to prevent lumps. Continue to cook, stirring frequently, until the sauce has thickened, about 6-8 minutes.

Add the Cheese and Seasoning: Remove the skillet from the heat. Add the shredded Gruyère and white cheddar cheeses (reserving a small handful for the topping), Dijon mustard, salt, pepper, and nutmeg. Stir until the cheese is completely melted and the sauce is smooth and luxurious.

Combine and Assemble: Pour the glorious cheese sauce over the Brussels sprouts in the mixing bowl. Add half of the crispy bacon bits. Gently fold everything together until the sprouts are evenly coated.

Bake the Casserole: Transfer the mixture to a 9×13-inch or other 3-quart baking dish. Spread it into an even layer. Top with the reserved handful of cheese, the remaining crispy bacon, and a sprinkle of Panko breadcrumbs (if using).

Bake to Perfection: Bake in a preheated oven at 375°F (190°C) for 20-25 minutes, or until the sauce is bubbly and the top is golden brown. For an extra-crispy top, you can place it under the broiler for the last 1-2 minutes, watching it carefully to prevent burning. Let the casserole rest for 10 minutes before serving.