Stir until smooth. Add in the cheese and stir to melt. Add the salt, seasoned salt and pepper. Pour the egg mixture into the sauce, whisking constantly.Take 1/4 cup of the sauce and slowly pour it into the beaten egg, whisking constantly to avoid cooking the egg.Don’t let it burn. Pour in the milk, add the mustard, and whisk until smooth.
Cook the mixture for 5 minutes, whisking constantly. In a large pot, melt the butter and sprinkle in the flour.The macaroni should be too firm to eat right out of the pot.