Place the mayonnaise, sour cream, lemon juice, parsley, fish sauce or Worcestershire, salt, pepper and (optional) scallions in a blender.
Blend until very smooth. If necessary to keep the blender moving, add milk or buttermilk by the half-teaspoonful.
Transfer the prepared dressing to a glass container with a tight-fitting lid, and store in the refrigerator until ready to use. If your sour cream is fresh, your dressing should keep for at least 2 weeks.
The dressing will thicken in the refrigerator, and become thinner again when brought to room temperature.
Make the pasta.
Cook the pasta to al dente texture, then drain the pasta water. Rinse the drained pasta with lukewarm water, drain it again, and toss it with the olive oil and salt. Cover the cooked pasta, and set it aside.
Assemble the pasta salad.
In a large mixing bowl, place the peppers, cucumber, and green onions, and mix to combine well.
Add the dressing, and mix well.
Add the mozzarella pearls, tomatoes, and Parmigiano-Reggiano, then the cooked pasta, and turn over gently to fully combine.
Cover the bowl, and allow the pasta salad to sit at room temperature for at least 1 hour before serving.
Before serving, add chopped flat leaf parsley, and serve at room temperature, with extra parsley and shredded Parmigiano-Reggiano cheese, to taste.