Cook your pasta, about one minute under the proposed al dente time. Drain, and place the pasta into a mixing bowl.
To make the cheese sauce, start by melting the butter in a sauce pot. Once melted, add in your flour, and stir for about 4 minutes or so with a wooden spoon. This is the start of your sauce. You want to cook the flour out, but not burn it. Slowly add in the milk, and continue to stir with a wooden spoon. You will keep stirring, yes, you will get a nice arm workout, until the sauce thickens. Once the sauce thickens, add one American cheese slice at a time until each slice melts. Add another slice, and repeat. Then, slowly add in the cheddar, one handful at a time. You want a smooth cheese sauce here. Add in the cayenne pepper. If you add too quickly, the cheese could get stringy, so just keep that in mind.
Pour the cheese sauce into the cooked noodles, and give a good mix.
Heat a large skillet on medium heat, then add in the oil.
Add in the sausage, and cook until lightly crisp. Toss in the onions, garlic, celery, and bell peppers. Cook until the onions sweat through but still have a bit of texture. I'm not going for overly soft vegetables on this one. Once cooked, add to the pasta bowl, and stir to combine everything.
This cajun style mac and cheese was awesome. It was perfectly cheesy, and when biting into it you get that great holy trinity along with the perfectly spiced, and texture andouille sausage nuggets. I've made some good mac and cheese in my time, but this one escalated to one of my favorites, plus it filled my hankering for that southern craving.