Cover the sun-dried tomatoes with hot water in a bowl. Set aside until soft, about 20 minutes, then drain, chop, and put in a medium bowl. To mellow the minced onion, soak it in cold water for 10 minutes, then drain it well, pat dry and add to the tomatoes.
Add the tuna, breaking it into large chunks, along with the celery, olives, and capers, and toss to combine. Add the olive oil, lemon zest and juice, and basil and toss again. Season, to taste, with salt and pepper. Serve cold, with bread or greens, if desired.
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