Mix the Parmigiano-Reggiano and bread crumbs together.
Add about a teaspoon full of the breadcrumb mixture into the muffin tins and dust it around to make sure the mixture sticks to the sides of the cavities as well as the bottom. Turn it upside down to get rid of any excess mixture. Set aside
Melt the butter in a medium sized pot.
Add the onion and cook on medium low heat until the onion has soften. Add salt and ground mustard and continue to cook until the onion is translucent. Do not brown.
Combine the half n half and the cheeses in the pot and cook until smooth and creamy. If adding ground mustard, add it now.
Remove from heat and taste to see if additional salt is needed. Continue to mix to help cool the mixture down.
Meanwhile put the pasta in a bowl and add the egg.
Add about 1/4 cup of the cheese sauce to the pasta & egg mixture and mix briskly.
Pour the rest of the cheese sauce over the pasta and mix well. If adding fresh herbs, add them here.
Spoon the pasta into the muffin tins.
If desired, sprinkle, evenly, some of the breadcrumb mixture on top of the mac and cheese for a nice crunchy top.
Bake for 15 minutes or until the top is starting to turn golden brown.
Remove from the oven and allow to rest for about 5 minutes before removing the mac ‘n’ cheese bites.