Bring a large pot of salted water to a boil, then cook the pasta until al dente, following package instructions. Drain and rinse with cold water to stop cooking and cool it down for mixing.
While the pasta cools, finely chop the red onion and celery, and set aside.
In a large mixing bowl, combine the drained pasta and flaked tuna, breaking up the tuna into smaller pieces with your fingers or a fork as you add it.
Add the chopped onion and celery to the bowl, mixing gently to distribute the vegetables evenly throughout the pasta and tuna.
In a small bowl, whisk together the yogurt, sour cream, lemon juice, salt, and pepper until smooth and creamy.
Pour the creamy dressing over the pasta mixture, then fold gently until everything is coated evenly and has a velvety appearance.
Taste and adjust seasoning with more salt, pepper, or lemon juice if desired.
Cover the bowl and refrigerate for at least 15 minutes to allow flavors to meld and improve the texture.
Remove from the fridge, give the salad a gentle toss, and serve chilled. The finished dish should have a smooth, velvety texture with bits of crunch from the vegetables and flaky tuna visible throughout.
