Bring a large pot of salted water to a boil then add the pasta and cook until al dente, about eight to ten minutes. Drain the pasta and leave it in the colander or strainer until ready to use.
Meanwhile, melt the butter in a saucepan over medium heat. Sprinkle the flour overtop along with the onion and mustard powders and whisk together. The mixture will be a little clumpy at first but will smooth out as you continue to whisk. Allow this to cook for about two minutes, stirring occasionally. Once the mixture starts to become a little golden, slowly whisk in the warm milk. Add in the salt and bring it to a light simmer. Cook, whisking frequently, until thick, about four to five minutes.
When the sauce is thick, turn the heat down to low and slowly add in the cheddar cheese. Switch to a heatproof spatula or a wooden spoon and carefully stir in the cooked macaroni making sure that every noodle is well coated.