Start by bringing a large pot of salted water to a boil. Carefully add the trimmed and halved Brussels sprouts, cooking them for about 5 minutes until they are bright green and just tender. Drain the sprouts and rinse them under cold water to stop the cooking process. Set aside.

In a large skillet, add the diced bacon and cook over medium heat for 7-10 minutes, stirring occasionally, until crispy and golden brown. Remove the bacon with a slotted spoon and place it on paper towels to cool, leaving 1 tablespoon of bacon fat in the pan.

In the same skillet, melt 2 tablespoons of butter over medium heat. Add the chopped onions and sauté for 5-7 minutes until they become translucent. Stir in the minced garlic, cooking for an additional minute until fragrant.

Sprinkle the flour into the pan with the onion mixture, stirring constantly for about 2-3 minutes. Gradually whisk in 2 cups of milk, followed by nutmeg, mustard, salt, and pepper. Cook until the sauce thickens, about 5 minutes, then stir in half the Parmigiano Reggiano until melted and smooth.

Add the blanched Brussels sprouts and crispy bacon into the creamy sauce, tossing gently to coat. Transfer this mixture into a greased 9x13 baking dish.

In a small bowl, mix the remaining Parmigiano Reggiano with panko bread crumbs. Sprinkle this topping evenly over the gratin.

Preheat oven to 350°F (180°C). Bake for 15-20 minutes, or until bubbly and golden brown. Broil for an additional 1-2 minutes if needed for extra crunch.