Cook pasta: Fill a large pot with water and bring to a rolling boil, then add a pinch of salt, add the rotini and cook according to package directions, usually 8 to 10 minutes, until al dente.
Cool completely: Drain the pasta in a colander and rinse thoroughly with cold water until completely cooled, about 2 to 3 minutes of running water, then drain well again to remove excess moisture.
Mix dressing: In a medium bowl, combine the Hidden Valley Dressing Mix powder with the mayonnaise and sour cream, then whisk vigorously for about 1 to 2 minutes until smooth with no lumps remaining.
Combine pasta and veggies: In a large serving bowl, add the cooled pasta, chopped bacon, diced red onion, cheddar cheese cubes, and halved cherry tomatoes.
Add greens: Gently fold in the romaine lettuce and avocado pieces just before serving to keep them fresh and prevent wilting.
Toss with dressing: Add the creamy dressing to the bowl and toss everything together until every piece is evenly coated, about 2 to 3 minutes of gentle tossing.
Finish: Sprinkle the chopped fresh parsley on top for a burst of color and fresh flavor, then serve chilled.