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 and set aside.

In a skillet over medium heat, add the sliced onion. Sprinkle with 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 gently until everything is well coated.

Chill the pasta salad in the refrigerator for at least 30 minutes before serving.

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