Combine Dressing Mix: In a medium bowl, whisk together the 1-ounce packet of Hidden Valley Dressing Mix with the 1 1/2 cups mayonnaise and 1/2 cup sour cream until smooth and well blended, about 1-2 minutes of whisking.
Taste and Adjust: Sample the dressing and add a pinch of black pepper or sea salt if desired for extra flavor depth.
Set Aside: Transfer the dressing to a container and refrigerate until you're ready to toss the salad, up to 2 hours ahead.
Cook and Cool the Pasta: Boil the rotini according to package directions, then drain and rinse thoroughly with cold water to stop the cooking process and remove excess starch, about 5 minutes of rinsing.
Crisp the Bacon: Cook all 13 slices of bacon in a skillet over medium-high heat until golden and crispy, roughly 8-10 minutes depending on thickness, then chop into bite-sized pieces.
Prep the Vegetables: Dice the red onion, tear the romaine lettuce, halve the cherry tomatoes, and cube the cheddar cheese, keeping each ingredient in its own small bowl or container until assembly.
Prepare the Avocado Last: Cut the avocado into small pieces just before mixing the salad to prevent browning, about 5 minutes before serving.
Start with the Pasta Base: Transfer the cooled pasta to a large mixing bowl, making sure it's completely cooled to room temperature.
Layer the Ingredients: Add the chopped bacon, diced red onion, cheddar cheese cubes, romaine lettuce pieces, cherry tomatoes, and avocado to the bowl with the pasta.
Add the Parsley: Sprinkle the 1/4 cup of chopped fresh parsley over the top for a bright, fresh finish.
Dress and Toss: Pour the prepared dressing over the entire salad and toss gently but thoroughly using a large spoon or salad tongs, making sure every ingredient is evenly coated, about 2-3 minutes of tossing.