Begin by preheating the oven to 400°F (200°C). In a large Dutch oven, bring 4 quarts of water and 2 teaspoons of salt to a boil over medium-high heat. Add the elbow pasta and cook for 8 minutes, or until al dente. Drain the pasta and set aside.

Heat the canola oil in a 12-inch cast-iron skillet over medium-high heat. Add the chopped Andouille sausage, yellow onion, green bell pepper, and red bell pepper. Sauté the mixture, stirring frequently, for approximately 8 minutes, or until the sausage begins to brown and the vegetables are tender. Add the minced garlic and cook for an additional minute. Remove the mixture from the skillet and set it aside. Wipe the skillet clean.

In the same skillet, melt the butter over medium-low heat. Gradually whisk in the flour and cook for 2 minutes, whisking continuously to prevent lumps. Slowly incorporate the milk while whisking, ensuring a smooth consistency. Increase the heat to medium and cook the mixture, whisking occasionally, until it thickens and begins to simmer, approximately 14–16 minutes. Stir in the Cajun seasoning and the remaining 1/2 teaspoon of salt. Remove the skillet from the heat and gradually mix in the shredded Cheddar and pepper jack cheeses until the sauce is smooth.

Gently fold the cooked pasta and the sausage-vegetable mixture into the cheese sauce, ensuring all components are evenly coated.

Transfer the combined mixture to a 9x13-inch baking dish, spreading it evenly. Place the dish in the preheated oven and bake for 20–25 minutes, or until the top is golden and the edges are bubbling. For a crispy topping, switch the oven to broil and cook for an additional 3–5 minutes, keeping a close watch to prevent burning.