Preheat oven to 350°F. Spray a 13x9-inch casserole dish with cooking spray.
Pasta: Cook the macaroni in a large pot of salted water per the directions on the package until al dente. Drain and set aside.
Make the Cheese Blend: Toss together the cheddar cheese and pepper jack cheese. Reserve 3 cups for the sauce and one cup for the topping.
Cook Sausage and Vegetables: Heat oil in a large 12-inch skillet over medium-high heat. To the pan add the andouille sausage, onion, green bell pepper, red bell pepper and celery. Cook, stirring often, until the sausage has begun to brown and the onion, peppers and celery are tender, about 8 minutes. Add garlic, and cook, stirring constantly, 1 minute. Transfer sausage mixture to a plate. Use paper towels to wipe skillet clean.
Make the Roux: Melt butter in skillet over medium heat. Whisk in flour, and cook, whisking constantly for 2 minutes. Gradually whisk in the milk. Increase heat to medium-high and bringing to a gentle boil. Immediately lower the heat and let gently bubble on low heat for 5 minutes to thicken.
To the roux add Creole mustard, Cajun seasoning and seasoned salt. Stir to combine, then add 3 cups shredded cheese stirring until melted and the sauce is smooth. Remove from heat. Gently stir in cooked pasta and sausage mixture.
Transfer the macaroni mixture to the prepared baking dish and sprinkle with the remaining one cup of cheese.
Bake until golden and bubbly, 35-40 minutes. Increase oven temperature to broil; broil until top is golden brown around, 3 to 5 minutes longer.
Let stand 5 minutes then serve immediately garnished with green onions.