Heat butter in a large pot over medium-high heat.
Add the onion and cook, stirring occasionally, until translucent, about 2 minutes.
Add the garlic and cook, stirring, for 30 seconds.
Add the flour and make a paste. This is the beginning of your roux.
Now, add the Dijon and stir to combine.
Add 1 cup of the milk to create a slurry. In this step, you just want the flour mixture to blend in so there are no clumps.
Add the remaining milk, broth, dry noodles, broccoli and carrots.
Bring to a boil and then reduce heat to maintain a simmer. Cook according to the cook time directions for al dente on the pasta package. Remember, you want to decrease cook time by about a minute if you are freezing the mac and cheese.
Once the noodles are cooked in the pot, remove from heat. It will be soupier than you want. This is very normal and what you want.
Stir in the cheese, 1 cup at a time, until evenly combined. At this point, you will see your mac and cheese tighten up.
Add salt and black pepper to taste.
Transfer the mac and cheese to your Souper Cubes trays.
Allow the mac and cheese to fully cool before you add the lid and transfer it to the freezer.
After 4 hours, you can pop out the cubes and transfer them to a freezer-safe container.
