Cook the fusilli pasta in a pot of salted boiling water until al dente as per package instructions. Drain and rinse under cold water, then set aside to cool completely.
In a large bowl, combine the drained tuna, cucumber, cherry tomatoes, and baby spinach. Toss gently to mix without breaking up the tuna too much.
In a separate bowl, whisk together the light mayonnaise and sour cream until well blended. Stir in fresh parsley and ground black pepper to taste.
Add the cooled pasta to the tuna mixture, then pour about half of the dressing over the salad. Mix gently to thoroughly coat.
Cover and refrigerate the salad for at least 30 minutes to allow the flavors to develop.
Before serving, add the remaining dressing and adjust seasoning if needed. Stir well and serve chilled.
Notes
The salad stays fresh for up to two days in the refrigerator if stored properly in an airtight container. Feel free to add other vegetables or herbs of your choice for extra flavor.