Cook the rotini pasta according to package instructions until al dente. Drain and rinse under cold water to cool. Toss with olive oil to prevent sticking.

In a skillet over medium heat, add the sliced onion, sugar, salt, and pepper. Cook, stirring occasionally, until the onions are caramelized and golden brown, about 15-20 minutes. Remove from heat and let cool.

In a large bowl, combine the mayonnaise, sour cream, Dijon mustard, and apple cider vinegar. Mix until smooth.

Add the cooled pasta, caramelized onions, shredded cheddar cheese, and chopped green onions to the bowl. Stir until everything is well combined.

Cover and refrigerate for at least 1 hour to allow the flavors to meld.

Before serving, sprinkle the crispy fried onions on top for added crunch.