Heat the oven to 350 F. Lightly grease a 2-quart baking dish.
Cook the pasta following making sure to follow the package directions. Drain well in a colander and rinse with hot water. Set aside.
In a skillet over medium heat with just enough oil to coat the bottom, cook the sausage with bell pepper until lightly browned and bell pepper is softened. Add green onion and garlic and cook for about 2 minutes longer.
Remove sausage mixture to a plate or bowl. Add the butter to the same pan and heat over medium heat until melted and frothy. Add the flour and stir until smooth. Cook, stirring for 2 minutes. Add Cajun seasoning and pepper. Gradually stir in the milk and cook, stirring until thickened and bubbly.
Add 2 cups of the cheese and cook, stirring until melted. Spoon into the prepared baking dish.
Top with the remaining cheddar cheese and bake for 20 to 25 minutes, until bubbly and the cheese is melted.