01 - Preheat oven to 400°F (200°C) and grease a 2-quart baking dish.
02 - Boil macaroni in a large pot of salted water until just al dente. Drain thoroughly and set aside.
03 - In a large saucepan over medium heat, melt butter. Whisk in flour and cook for 1 minute. Gradually add milk and heavy cream, whisking constantly, until the mixture thickens, about 4–5 minutes.
04 - Add grated cheddar, Gruyère, Dijon mustard, paprika, salt, and black pepper. Stir until the cheese is melted and the sauce is smooth.
05 - In a large mixing bowl, combine drained macaroni, cheese sauce, chopped lobster, chives, lemon zest, mayonnaise, lemon juice, and tarragon if using. Gently fold until fully mixed.
06 - Spoon the mixture evenly into the prepared baking dish.
07 - Combine panko breadcrumbs, melted butter, parsley, smoked paprika, and sea salt in a small bowl. Sprinkle mixture evenly over the top.
08 - Bake for 20–25 minutes until the topping is golden and edges are bubbling.
09 - Allow to rest for 5 minutes before serving. Garnish with additional chives if desired.