Mac and Cheese with Smoked Sausage

14-16 ounces fully-cooked smoked sausage, cut into medium chunks
8 ounces dried elbow macaroni
2 tablespoons unsalted butter
1/2 cup finely chopped onion
2 tablespoons flour
1/2 teaspoon dry mustard
1/8 teaspoon cayenne pepper
2-1/4 cups 2% milk
1 cup (4 ounces) shredded sharp Cheddar cheese
1 cup (4 ounces) shredded Monterey Jack cheese
2 teaspoons Dijon mustard

1. In a large nonstick skillet, sauté smoked sausage, over medium heat, until very lightly browned.  Remove with a slotted spoon, drain and transfer to a bowl.
2. Meanwhile, over high heat, and in a large, heavy saucepan, bring 2 to 3 quarts cold water to a rolling boil.  Add pasta and stir.  Return water to a boil and reduce heat.  Stir the pasta occasionally.  Following package directions, cook just until the macaroni is tender.  Drain pasta.
3. In the same saucepan, melt butter over medium heat.  Add onion and sauté just until the onion is soft.  Whisk in flour, dry mustard and cayenne pepper, blending well.  Continue to cook and stir for 1- 2 minutes or until the flour is slightly golden.  Slowly pour in milk, whisking all the time.  Continue to cook and stir until the mixture thickens slightly and comes to a simmer.  Continuing to stir and cook another 1-2 minutes.
4. Remove the saucepan from the heat and quickly add cheeses, stirring until the cheeses melt.
5. Return the saucepan to medium heat and stir in prepared mustard, smoked sausage and cooked macaroni.  Stir to coat the sausage and pasta with the cheese sauce.  Continue to heat until the mixture is steaming and heated throughout.
6. Serve immediately.