Bring a large pot of salted water to a rolling boil. Cook the rotini pasta according to package directions until al dente, typically 8-10 minutes.
Drain the pasta thoroughly in a colander and rinse immediately with cold water to stop the cooking process and prevent sticking. Shake off excess water and transfer to a large mixing bowl.
Add the cooked shrimp, crab meat, halved cherry tomatoes, diced cucumber, and sliced black olives to the bowl with the cooled pasta. If using optional red onion, add it now.
In a small bowl, prepare the dressing by whisking together the olive oil, fresh lemon juice, and Dijon mustard until well combined. Season with salt and freshly ground black pepper to taste.
Pour the dressing over the pasta salad ingredients and gently toss until everything is evenly coated.
Cover the bowl and refrigerate for at least 30 minutes (or up to 2 hours) to allow the flavors to meld together.
Just before serving, give the salad another gentle toss and garnish with the freshly chopped parsley.
Serve chilled for the best flavor and texture.
