01 - Preheat oven to 375°F. Butter a 2-quart baking dish thoroughly.
02 - Bring salted water to boil in large pot. Cook pasta until al dente, about 1-2 minutes less than package directions. Drain and set aside.
03 - Melt 3 tbsp butter in large saucepan over medium heat. Whisk in flour and cook, stirring constantly, for 2 minutes until fragrant but not browned.
04 - Gradually whisk warm milk into roux, about ½ cup at time, smoothing any lumps. Continue whisking until sauce thickens, 4-5 minutes. Sauce should coat back of spoon.
05 - Remove from heat. Stir in Gruyère, cheddar, and Parmesan until completely melted and smooth. Add mustard powder, cayenne, salt, and black pepper.
06 - Fold cooked pasta and lobster into cheese sauce, mixing gently to avoid breaking lobster pieces. Transfer mixture to prepared baking dish.
07 - In bowl, combine breadcrumbs, melted butter, parsley, chives, lemon zest, and pinch of salt. Mix until evenly coated.
08 - Sprinkle herbed breadcrumbs evenly over macaroni and cheese. Bake 20-25 minutes until top is golden brown and sauce bubbles around edges.
09 - Let dish rest 5 minutes before serving to allow sauce to set for easier plating.