Spray a shallow 2 qt. baking dish with cooking spray.
Boil a large pot of water and cook pasta until al dente.
Drain, reserving the pasta water for later.
Melt 4 Tbsp butter in a large pan over medium heat. Add in flour and whisk to combine and cook until it turns light brown.
Add in milk, whisking as you pour. Stir until smooth.
Add in seasoning and cook until sauce thickens.
Add in cheeses, one at a time and stir until each cheese is melted before adding next cheese.
Add in pasta. Use remaining pasta water to thin sauce if it has thickened too much.
Place mixture into greased dish.
Melt remaining 2 Tbsp butter and add the breadcrumbs, Parmesan and a pinch of salt. Stir to combine
Sprinkle over top of pasta and bake until browned and bubbly. About 15-20 minutes.
Garnish with sliced green onions.
Notes
Cheese. I used one of my favorite cheese combo in this recipe but you could also use Pepper Jack Cheese, Sharp Cheddar Cheese, Mozzarella Cheese or Colby Jack. Come up with your own favorite combo of cheeses.
Shred your own cheese. I recommend NOT using pre shredded cheese. Pre-shredded cheese may have an anti-caking agent on it that will prevent it from melting smoothly. You also get a better flavor from freshly grated cheese.
Breadcrumbs. Don't skip this part, it adds a nice texture contrast to the creaminess of the mac and cheese.
Dress it Up. To make it even better, add in some crispy, cooked bacon!