If using frozen shrimp, thaw it in the refrigerator. Pat dry. Cook raw shrimp in a skillet over medium heat for approximately 2–3 minutes per side, until opaque and cooked to 145°F. Cool completely. If using pre-cooked shrimp, thaw and pat dry without recooking.
Cook tortellini according to package directions until tender but still firm. Drain thoroughly, briefly rinse with cool water, and drain again. Cool.
Dice the cucumber, bell pepper, and red onion. Halve the tomatoes and roughly chop the spinach.
In a small bowl, whisk together Greek yogurt, light mayonnaise, lemon juice, Dijon, dill, garlic, salt, and pepper. Add cold water a teaspoon at a time if needed.
Place the cooled tortellini and shrimp in a large bowl. Add the cucumber, tomatoes, bell pepper, onion, and spinach.
Add most of the dressing and gently fold until everything is coated. Add the remaining dressing only if needed.
Cover and refrigerate for 20–30 minutes. Taste, adjust seasoning, and divide into six equal servings.