Preheat oven to 400°F. Bring water and 2 teaspoons of the salt to a boil in a Dutch oven over medium-high. Add pasta; cook 8 minutes. Drain and set aside.

Heat oil in a 12-inch cast-iron skillet over medium-high; add sausage, onion, green bell pepper, and red bell pepper. Cook, stirring often, until sausage has begun to brown and onion and peppers are tender, about 8 minutes. Add garlic, and cook, stirring constantly, 1 minute. Transfer sausage mixture to a plate; wipe skillet clean.

Melt butter in skillet over medium-low; whisk in flour, and cook, whisking constantly, 2 minutes. Gradually whisk in milk. Increase heat to medium, and cook, whisking occasionally, until mixture thickens slightly and comes to a simmer, 14 to 16 minutes. Continue to cook, whisking constantly, 5 minutes. Whisk in Cajun seasoning and remaining 1/2 teaspoon salt; whisk in cheeses until smooth, and remove from heat. Gently stir in cooked pasta and sausage mixture.

Transfer to preheated oven with oven rack 4 to 6 inches from heat. Bake until golden and bubbly, 20 to 25 minutes. Increase oven temperature to broil; broil until top is browned and crisp, 3 to 5 minutes.