Cook the macaroni according to the package directions. Drain well and transfer the hot macaroni to a large bowl or lidded container.

While the macaroni is still hot, sprinkle the apple cider vinegar over the noodles.

Add the shredded carrots and onion, if using, and toss until well combined.

Refrigerate for 15–20 minutes, until the macaroni has cooled.

In a separate bowl, whisk together the mayonnaise, milk, and sugar until smooth.

Pour the mayonnaise mixture over the cooled macaroni and gently fold until all the noodles are evenly coated.

Season with kosher salt and black pepper to taste.

Cover and refrigerate for at least 4 hours before serving. Even better made the night before.

Stir well before serving. If it seems too thick or dry, stir in another tablespoon or two of milk until creamy again. Serve cold.