Preheat the oven to 400 degrees.
Fill a Dutch oven with salted water. Bring to a boil over high heat.
Add the elbow macaroni and cook for 8 minutes. Drain the macaroni and set aside.
Return the empty Dutch oven to the stovetop. Add the oil and heat over medium-high heat.
Add the sausage, bell peppers, and onion. Cook, stirring often, for 6 to 8 minutes or until tender. Stir in the garlic.
Remove the sausage and vegetables from the Dutch oven and wipe the pot to remove any excess grease.
Reduce the heat to medium. Add the butter to the Dutch oven and melt.
Whisk in the flour and cook for 2 minutes, stirring constantly.
Slowly add the milk, whisking well. Cook, stirring often until the sauce begins to thicken. Add the seasoning.
Stir in about 1 ½ cups of each cheese, mixing until completely melted.
Add the macaroni, sausage, and vegetables to the mixture, stirring until combined.
Smooth the top of the casserole and add the remaining cheese on top.
Bake for 20 minutes or until the cheese is melted on top and the edges are bubbling.