Preheat oven to 325°F. Spray a large 9x13-inch baking dish with non-stick cooking spray and set aside until ready to use.
Heat four quarts of water in a large pot over high heat. Add one tablespoon of Kosher salt once the water comes to a boil. Add the pasta and cook al dente for minus two minutes. About a minute before the pasta is done, reserve two cups of water to add to the cheese sauce. Set aside until ready to use. Drain the pasta in a colander set in your sink, discarding the remaining pasta water.
After heating the pasta's water, place a large Dutch oven over medium heat. Add the sliced andouille sausage and cook, stirring frequently, for about 10 minutes. Remove from the pot and place on a paper towel-lined plate, then place more paper towels on top to adsorb grease from the sausage.
In the same pot you cooked the sausage in, melt salted butter. Once the butter has melted, stir in minced garlic and cook for 30 seconds, stirring constantly. Add flour and whisk constantly for about three minutes to make a roux. Make sure to scrape the bottom of the pot to remove any of the bits, or fond, from the bottom.
Slowly pour in room temperature half and half of the mixture into the room temperature, whisking constantly to form a smooth mixture. Making sure to continue scraping the bottom of the pot as you whisk.
Add the Dijon mustard and Cajun seasoning. Continue whisking constantly until the mixture begins to simmer. Do not boil; this will cause the sauce to become grainy.
Reduce heat to low. Add four ounces each of freshly shredded Gruyère, Gouda, and Extra-Sharp Cheddar cheese. Whisk until the cheeses have melted and the mixture is well combined. Add reserved pasta water, about ½ cup at a time, whisking well after each addition, to emulsify the sauce fully. I added a total of one cup, but you may need to add more. If you need to add more than one cup, only add ¼ cup at a time; you don't want the sauce to be too watery.3
Place the cooked and drained pasta into a large stainless steel bowl, along with the cooked andouille sausage and crawfish tails, and stir with a silicone spatula to combine everything. Then, pour the sauce into the bowl and stir to combine.
Use a large ladle to layer about half of the pasta mixture into the bottom of the prepared dish. Then, sprinkle two ounces each of Gruyère, Gouda, and Extra-Sharp Cheddar cheeses over the top. Add the remaining pasta mixture on top of the sprinkled cheese. Then, sprinkle the remaining cheeses on top.
Bake in 325°F for 25-30 minutes, until hot and bubbly. Remove from the oven and allow to cool for about five minutes before serving. Sprinkle with some freshly minced parsley to garnish.
Yield 8 servings