Heat the oven to 425°F. Lightly butter a 3-quart ovenproof dish and set aside.
If using stale bread, tear it into 1-inch pieces and pulse them in a food processor until smooth. Melt 2 Tbs. of the butter in a skillet over medium heat, add the breadcrumbs, and swirl to coat with butter. Cook the breadcrumbs for 2 minutes, or until browned. Season with salt and pepper and set aside.
In a large saucepan, melt the remaining 5 Tbs. butter over medium-low heat. Whisk in the flour and cook, whisking constantly, for 2 minutes to make a roux. Whisk in the milk in a slow, steady stream, whisking constantly for about 3 minutes. When it comes to a simmer, stir in the cheese, nutmeg, and mustard; season with salt and pepper to taste. Remove from the heat and cover.
Boil a large pot of water, season with salt, and cook the pasta as directed on the package instructions. Drain the pasta and pour into a large bowl. Add the cheese sauce and lobster meat; mix well. Transfer to the prepared dish. Sprinkle the breadcrumb topping evenly over the macaroni. Bake, uncovered, for 20 to 30 minutes, or until golden brown. Let cool for 5 minutes before serving. Garnish with the parsley and serve.