Preheat oven to 375°.
Bring a large pot of water to a boil and cook pasta until lightly firm. Drain pasta and set aside.
To defrost lobster, empty contents of the bag in a bowl of cold water and let sit for 15-20 minutes.
Meanwhile, melt 3 tablespoons of butter in a large saucepan over medium heat. Add chopped onion and let cook for 2 minutes or until onions become soft. Add flour and stir continuously for 1 minute.
Add milk and whisk constantly until mixture comes to a boil. Set aside 1/4 cup of cheeses combined and add the rest to the milk mixture. Add Dijon mustard and turn heat to low, stirring until all cheese is fully melted. Once melted, remove from heat entirely and stir in salt and pepper.
Fold in macaroni and lobster meat and pour into a large baking dish.
In a separate bowl, combine panko breadcrumbs, grated Parmigiano Reggiano, and remaining cheese and butter (1 tablespoon). Sprinkle evenly over dish and let cook for 20 minutes or until breadcrumbs start to brown.
Enjoy!