Drain the liquid from the can of tuna, then turn tuna into a bowl, breaking it neatly into fairly large chunks.
Add the olives, mushrooms, artichokes, onions, and the giardiniera, all previously drained of their liquids.
Then add the tomato sauce, oil, vinegar, and salt.
With 2 forks, toss to blend together well, being careful not to mash the chunks of tuna.
For best flavor, allow the mixture to stand for several hours or overnight in the refrigerator.