Preheat oven to 350 degrees F.

In a large stock pot over medium heat, add butter and onion. Cook for 5 minutes until onion is translucent.

To the onion and butter mixture, add flour and stir well to incorporate. Cook for 2 minutes, then add milk, stirring well again. Cook over medium heat, stirring often, until mixture comes to a simmer. Simmer for 2-4 minutes, or until thickened. Remove from heat and add ground mustard, 1 ½ tsp of the seasoned salt, black pepper and all cheeses minus 1 cup of the shredded cheeses which will be used for garnish. Stir or whisk well until smooth. Add sweet corn, bacon and chives if desired. Set aside.

Boil pasta for 2 minutes less than box instructions say. Strain pasta, reserving 1 cup of pasta cooking liquid. Add pasta to large stock pot with cheese/corn/bacon sauce. Stir well. Add pasta cooking liquid if needed so the sauce is not too thick. It will continue to cook during the bake time, so it needs to be a bit looser to start.

Spray a 9x13 pan with cooking spray. Add cheese sauce covered pasta. Sprinkle reserved shredded cheese on top. Sprinkle with ½ tsp seasoned salt.

Bake for 15 minutes at 350 degrees F. For a browned top, broil on high for 2-4 minutes until top of mac and cheese is browned. Serve warm.