If using whole cooked lobsters, crack and remove all meat from tails, claws, and knuckles. Chop the lobster meat into 1/2 to 3/4-inch pieces. Place in a bowl.

In a small pan, melt 2 tablespoons of butter. Add the chopped lobster meat and chives, stirring gently for 1-2 minutes over low heat, just to warm through and infuse flavor. Do not overcook. Set aside.

Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish.

Bring a large pot of heavily salted water to a rolling boil. Add the macaroni and cook according to package directions until al dente (about 1 minute less than package suggests). Drain well and set aside.

In a large saucepan or Dutch oven, melt 6 tablespoons of butter over medium heat. Add the flour and whisk constantly for 2-3 minutes to create a smooth roux. It should be a pale blonde color.

Gradually whisk in the warmed whole milk and heavy cream, a little at a time, until smooth and no lumps remain. Bring the sauce to a gentle simmer, whisking occasionally, until it thickens, about 5-7 minutes.

Remove the saucepan from the heat. Stir in the dry sherry (if using), Dijon mustard, nutmeg, white pepper, and salt. Add the grated cheddar, Gruyere, and Fontina cheeses, stirring until completely melted and smooth. Taste and adjust seasoning as needed.

Add the cooked macaroni to the cheese sauce and stir to coat evenly. Gently fold in the warmed lobster meat.

Pour the macaroni and cheese mixture into the prepared baking dish, spreading evenly.

In a small bowl, combine the Panko breadcrumbs, grated Parmesan cheese, and 2 tablespoons of melted butter. Sprinkle this mixture evenly over the top of the mac and cheese.

Bake for 25-30 minutes, or until the sauce is bubbly and the topping is golden brown and crispy.

Remove from the oven and let rest for 5-10 minutes before serving. This allows the sauce to set slightly.