Prepare pasta as directed on package, but cooking only 6 minutes; drain.
In a large pot, melt butter over medium heat. Add onion and cook until translucent. About 5 minutes.
Stir in arrowroot powder.
Gradually stir in milk. Make sure you get all the bits off the bottom!
Cook and stir until mixture thickens and comes to a boil. It should coat the back of the spoon - it should be that thick! And be careful not to scald the milk. Turn the heat down if it is too hot.
Stir in cheese, mustard, salt & pepper and mix well until all cheese is melted.
Mix pasta and sauce together (you can either add it back to the cheese sauce pot or using a separate large bowl) and pour into 13x9-inch baking pan.
Bake for 35 minutes or until lightly browned.
Let cool for 5 minutes then dig in!
Notes
If you want a crispier top, turn on your broiler and broil for about 5 minutes after baking time has finished. Be sure to watch it closely though so it doesn't burn.
Recipe by Pastured Kitchen at https://www.pasturedkitchen.com/recipes/moms-mac-n-cheese/