In a medium bowl, whisk together the half-and-half, mayonnaise, grated onion, dark brown sugar, salt, and pepper until smooth. Refrigerate until needed.

Cook the macaroni according to the package instructions, adding 5 extra minutes for tenderness. Drain and return to the pot, then stir in the apple cider vinegar. Let cool for 20 minutes.

Add half of the dressing to the macaroni and stir to combine, then refrigerate for another 20 minutes.

Stir in the remaining dressing along with green onions, celery, and grated carrots. Adjust seasoning if needed and cool for 30 minutes.

Transfer the salad to a bowl, cover with plastic wrap, and chill in the refrigerator for at least 2 hours before serving.