Fill a Pasta Insert Set with water and bring to a boil. Season water generously with salt, add pasta, and cook according to package directions until just al dente (avoid overcooking). Drain pasta and set aside.

In a 3 QT Saucier or 3.5 QT Non Stick Saute Pan over medium heat, melt the butter. Sprinkle flour over butter and use a Wooden Spoon to incorporate, stirring to create a roux. Cook, stirring constantly, until roux is thickened and light golden but not brown, 3-4 minutes.

Gradually pour in milk, whisking constantly to incorporate the liquid and create a béchamel sauce. Raise heat to medium-high and bring sauce to a low boil, whisking constantly to avoid any lumps. Reduce to a simmer over low heat, whisking occasionally, and cook until béchamel sauce is thick and coats the back of a spoon, 3-5 minutes more.

Add gochujang, 1 ½ tsp. salt, and mustard powder, if using. Add the cheese in three batches, whisking until each addition is completely melted before adding more. Remove from heat. Taste and adjust seasoning, adding more salt or gochujang as needed.

Add the drained pasta to the pot with the cheese sauce and stir well to coat. Fold in half of the sliced scallion. Divide among bowls and top with remaining scallion.