Remove the jalapeños and vegetables (typically carrots and onions) that have been packed with them; reserve the liquid. Cut the stems off 2 to 3 of the jalapeños, cut them in half and scrape the seeds out; discard stems and seeds. Thinly slice the chiles (you need about 1/4 cup). Thinly slice some of the vegetables (you need about 1/4 cup of these also).
In a large skillet, heat the olive oil over medium. When hot, add the onion and cook until richly golden, about 10 minutes. Remove from the heat and stir in 1/4 cup of the jalapeños pickling juice. Let cool. Stir the onions into the tuna, along with the sliced jalapeños and vegetables and the parsley or herbs. Set out for your guests to roll into warm tortillas to make soft tacos.