01 - Preheat oven to 375°F. Butter a 2-quart baking dish.
02 - Cook pasta in salted boiling water until just al dente; drain and set aside.
03 - In a large saucepan over medium heat, melt 3 tbsp butter. Whisk in flour and cook for 2 minutes.
04 - Gradually whisk in warm milk, stirring constantly. Cook until thickened, about 4–5 minutes.
05 - Remove from heat. Stir in Gruyère, cheddar, and Parmesan until smooth. Add mustard powder, cayenne, salt, and black pepper.
06 - Fold in cooked pasta and lobster, mixing gently. Transfer mixture to prepared baking dish.
07 - In a bowl, combine breadcrumbs, melted butter, parsley, chives, lemon zest, and salt.
08 - Sprinkle herbed breadcrumbs evenly over the macaroni and cheese.
09 - Bake for 20–25 minutes, or until top is golden and sauce is bubbling.
10 - Let rest for 5 minutes before serving.