Cook pasta according to package directions - be sure to salt the boiling water. Drain well.
While the pasta is cooking, cook the sausage over medium heat in a large pot (at least 5 Qt). Use a wooden spoon to break up the sausage into small bits. This takes about 5-10 minutes. Once the sausage is cooked through, remove it to a plate or bowl and set aside.
Melt the butter in the sausage pan over medium heat. Add the onion and cook until soft, about 2-3 minutes. Add the garlic and cook 1 more minute. Stir in the flour completely and cook for 1 minute, then add the milk. Stir to combine the mixture, then add the italian seasoning, pepper, and dry mustard.
Cook over medium heat, stirring constantly, until the mixture thickens, about 10 minutes or so. It should be the consistency of a creamy soup. Stir in the cheese until melted. Add salt to taste (and more pepper, if desired).
Add sausage and pasta into the cheese sauce and stir until combined. Remove from heat and serve hot.
Store in an airtight container for up to 3 days. This can be made one day ahead and reheated in the oven as well.