Boil water in a big pot, add a pinch of salt, and cook 16 oz of macaroni until it's almost soft but still has a little bite (al dente). Then drain it.
In a small bowl, whisk together 1.5 cups of milk and 1/2 cup of sourdough discard.
Melt 1 stick of butter in a large pan over medium heat.
Stir in 1/4 cup of flour until smooth.
Gradually add 2 cups of heavy cream, keep stirring, then pour in the milk and starter mix. Keep whisking until it thickens.
Take the pan off the heat and slowly mix in 8 cups of shredded cheese (like sharp cheddar and white cheddar), a little at a time, until it's all melted and smooth.
Add the drained pasta to the cheese sauce, stirring well to coat all the pasta.
Pour the mac and cheese into a 9x13 inch baking dish.
Sprinkle bread crumbs and parmesan on top.
Bake at 350°F for about 20 minutes or until the top is golden brown.
Once baked, let the Sourdough Discard Cheddar Mac and Cheese sit for a few minutes before serving. This rest period allows it to set slightly, making it easier to serve.