Affectionately nicknamed "Junkyard" Mac 'n Cheese as our station happens to be right next to an old Brooklyn junkyard, and these three cheeses in action together wound up having the mac disappear from the fridge in record time. Hopefully you'll enjoy it just as much.
Preheat oven to 350 degrees Fahrenheit. Grease a 2-quart casserole dish with cooking spray or an extra 1/2 tbsp of butter and set aside.
In a large pot, bring water to boil, salt, and add uncooked pasta. Cook, stirring occasionally, until just underdone (not al-dente, but maybe a minute or two before that point). Drain, put aside.
While waiting for water to boil and pasta to cook, finely chop or mince onion.
Grate and measure out three cheeses listed, collect in a medium bowl and put aside. If you want to save time, you can buy cheeses shredded, but you will have less control over the quality and strength of the cheese.
Measure out one cup of milk while still cold and mix in all dry ingredients (flour, salt, pepper, mustard seed) save for the bread crumbs until well incorporated and no lumps are evident.
In a small saucepan, melt remaining 2 tbsp of butter and then immediately remove from heat. Stir in breadcrumbs until all breadcrumbs are coated.
In a large saucepan, melt 1 tbsp of butter over medium-low heat. Add onion, stir frequently until onion becomes translucent (3-5 minutes).
Add 1/2 cup of remaining milk. Allow milk to warm, but not boil. Add 1 cup of milk that has dry ingredients mixed into it to saucepan mixture. Mix for about 10-15 minutes to allow for thickening.
Over extremely low heat, slowly add cheese from bowl in several parts, stirring and making sure cheese is melted before adding the next part. Once all cheese is in saucepan, remove from heat, and continue to mix until sauce is smooth and fully melted.
Pour reserved pasta into casserole dish. Cover with cheese sauce. Top with buttered bread crumbs. Place in oven and cook for about 20 minutes, or until the top is bubbling and the crumbs are a deep golden brown.