Cook the rotini pasta according to package instructions until al dente. Drain and rinse under cold water to cool.

In a large skillet, heat the olive oil over medium heat. Add the sliced onion, sugar, salt, and pepper. Cook, stirring frequently, for about 10-15 minutes until the onions are caramelized and golden brown. Remove from heat and let cool.

In a large mixing bowl, combine the cooled pasta, caramelized onions, garlic powder, mayonnaise, sour cream, Parmesan cheese, parsley, and green onions. Mix well until all ingredients are evenly coated.

Cover the bowl and refrigerate for at least 1 hour to allow the flavors to meld. Serve chilled or at room temperature.