Drain the tuna thoroughly, pressing it with a fork against the lid or in a sieve. Excess water makes the salad runny.
In a medium bowl stir together the yogurt, mayonnaise (if using), mustard, lemon juice and zest, garlic powder and dill.
Add the tuna and break it into flakes with a fork. Fold in the cucumber, celery and red onion. Season with salt and pepper and taste; add more lemon if it needs brightness.
In a separate bowl toss the greens with the olive oil, vinegar and a pinch of salt.
Divide the greens between two bowls. Top with the corn, shredded carrot and tomatoes, then pile the tuna salad in the middle. Finish with black pepper and extra dill.