Pre-heat oven to 375F
In a pot of boiling, salted water cook macaroni for about 9 minutes. It will still be quite al dente, but will finish cooking in the oven later. Drain macaroni and set aside.
In large pot, melt butter over medium-low heat. Add 1/2 Tablespoon of flour, whisk until combined. Repeat. There should be no lumps!
Slowly add evaporated milk. I add about 2 ounces at a time, whisk for 8-10 seconds, repeat. This will ensure a thicker sauce. If you dump all the milk at once, the roux will be very thin
Add salt, pepper, mustard, onion and garlic. Whisk well to combine.
REMINDER: for onion & garlic. You will use either fresh OR powder, not both!
Add cheese to roux, and whisk until cheese is fully melted. Continue stirring slowly and constantly for another 2-3 minutes
CHEESE SAUCE CONSISTENCY: you want the cheese sauce to have a rich consistency. The sauce should hold to the back of a spoon and, when you swipe down the spoon with your finger, the sauce does not come back together
Add macaroni to cheese sauce, and mix well.
Spray baking dish generously with cooking spray. Add macaroni and cheese sauce. Sprinkle with corn flake crumbs, plus a little bit of cheese. If desired, add a few bits of butter
Bake for about 30 minutes, or until the sauce is set. Remove from oven, and allow to cool for about 5 minutes. Enjoy!