Preheat over to 400.
Cook macaroni about 1 minute less than the directions call for. Drain, rinse with cold water, and set aside.
While pasta is cooking, melt butter over medium heat in a large saucepan. Remove 3 tablespoons of melted butter to a bowl; set aside. Add onion and garlic; cook until browned. Add flour; cook 1 minute while stirring. Add heavy cream and milk; whisk until smooth. Bring to a boil. Reduce to a simmer; cook until sauce has thickened, about 2 to 3 minutes. Stir in 3/4 teaspoon salt and 1/2 teaspoon pepper.
Remove from heat. Fold in cheese, lobster, and macaroni. Place in shallow 4-quart baking dish. To bowl of butter, stir in breadcrumbs; sprinkle over macaroni. Bake uncovered until topping is golden, 15 to 20 minutes. Let cool 5 minutes before serving.