Step 1: Cook the Macaroni: Bring a large pot of salted water to a boil. Add the elbow macaroni and cook according to package directions until al dente, usually about 7-8 minutes. Avoid overcooking, as the macaroni will continue to soften in the dressing.
Step 2: Drain and Cool: Drain the cooked macaroni in a colander and rinse it thoroughly with cold water to stop the cooking process. This step is crucial for preventing the macaroni from sticking together. Allow the macaroni to cool completely before proceeding to the next step.
Step 3: Prepare the Dressing: In a large bowl, whisk together the mayonnaise, milk, apple cider vinegar, sugar, Dijon mustard, salt, and pepper until smooth and well combined. Adjust the sugar, vinegar, and salt to taste, ensuring a balance of sweetness and tang.
Step 4: Combine and Mix: Gently fold the cooled macaroni into the dressing until it is evenly coated. If using, add the finely grated onion at this point. Be careful not to overmix, as this can cause the macaroni to become mushy.
Step 5: Chill and Serve: Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or preferably overnight, to allow the flavors to meld. This chilling period is essential for developing the signature taste and texture of Hawaiian macaroni salad. Serve cold as a side dish.
Optional Tips: For a creamier salad, add an extra tablespoon or two of milk. If a sweeter taste is desired, increase the amount of sugar or add a tablespoon of sweet pickle relish. For a tangier flavor, add a bit more apple cider vinegar. Some people also add shredded carrots or celery for added crunch and color.