Fill a medium saucepan with 4 to 6 cups of water and bring to a boil over high heat. Add the pasta from the box of macaroni and cheese; set the powdered cheese packet aside. Cook the pasta according to the package instructions (about 7 to 10 minutes).
In a medium saucepan, combine 1 tablespoon butter, seasoned salt, sausage and minced onion. Cover and warm over medium-low heat, stirring occasionally.
Put the fish into a strainer and rinse with hot water. Add the rinsed fish to the pan with the sausage along with the mustard and soy sauce. Stir to combine, cover and continue to cook until warmed throughout, about 5 to 10 more minutes.
When the pasta is cooked, drain and place in a medium mixing bowl. Add the remaining butter, the milk, the reserved packet of powdered cheese and the cheese to the pasta and stir to combine. The mixture should be thick and creamy.
Add the sausage and mackerel mixture to the pasta and mix well. Top with peppers, if desired, and enjoy.