Wash the corn and beans in a colander and drain. Clean the celery, wash it well, dry it and cut it into slices, finely chop the leaves. Drain the tuna and chop it up. Wash and dry the apple, remove the seeds, stems and blossoms and dice. Mix the lemon juice with salt, pepper and oil, mix in the prepared ingredients and let stand for 30 minutes.
Mix the remaining ingredients into the sauce and season with salt and pepper. Arrange the salad, cover with the sauce and sprinkle with celery leaves.