For an even richer flavor, use a blend of Cheddar and Gruyere cheese. Don't overcook the macaroni; al dente is key for the best texture. Adjust the amount of mustard and garlic salt to suit your taste preferences. Feel free to experiment with other toppings, such as cooked broccoli florets, diced ham, or crumbled blue cheese. To make ahead, prepare the mac and cheese and toppings separately. Store in the refrigerator and reheat before serving.