Preheat your oven to 425ºF and lightly grease a 23 centimeter square casserole dish with non-stick cooking spray or butter.
Bring a large pot of salted water to a boil then add the pasta and cook until al dente, about eight to ten minutes. Drain the pasta and leave it in the colander or strainer until ready to use.
Meanwhile, melt the butter in a saucepan over medium heat. Add in the garlic and cook for about one minute then sprinkle in the flour. Whisk the flour in and cook, whisking constantly, for about one to two minutes. While continuing to whisk, slowly pour in the warm milk, salt, pepper, and nutmeg and bring it all to a simmer. Cook, whisking frequently, until thick, about four to five minutes.
When the sauce is thick, turn the heat down to low and whisk in the mustard, grated Gruyere, and grated cheddar. Switch to a heatproof spatula or a wooden spoon and carefully stir in the cooked rigatoni along with 1/4 cup of finely chopped chives, making sure that every noodle is well coated. Pour half of the mixture into your prepared casserole dish, scatter on half of the lobster, then layer on the remaining pasta and lobster and set aside.
To make the topping, whisk the egg and remaining two tablespoons of milk in a bowl. Season with salt and pepper and stir in the torn bread, one tablespoon of finely chopped chives, and the Parmigiano Reggiano cheese.
Top the casserole with this bread mixture and pop it in the oven for 20 to 25 minutes or until golden brown and bubbly. Allow the mac and cheese to rest for a few minutes before digging in!