Combine the oil, onion, herbs, peppercorns, lemon zest, and salt in a deep sauté pan or Dutch oven. Heat to between 140° and 150°F, stirring occasionally, and cook for 20 minutes to infuse the flavors of the aromatics into the oil and to pasteurize it for a long shelf life. Taste the oil; it should be slightly salty. Leave to cool and infuse for about 30 minutes; the oil will be warm.
Put the pan back over medium-low heat and slip the tuna into the barely warm oil. (Add as many pieces as will fit in one layer. The tuna must be covered by the oil; add more if needed.) Slowly bring the oil to 150°F again. Turn off the heat, take the pot from the heat, and let the tuna cook slowly in the warm oil. After a minute or two, test for doneness by breaking into the flake of the tuna. The fish should be cooked to medium rare-slightly pink inside and still tender to the touch. If the tuna isn't quite done, return it to the oil for another minute. Repeat with any remaining pieces of tuna.
Transfer the tuna to a storage dish (I prefer glass or crockery, but an airtight plastic container will do fine) and let it cool. Let the oil cool separately and then strain the oil over the fish, discarding the aromatics. If the tuna isn't completely covered in oil, add more fresh olive oil to the storage dish. If not using right away, cover the container tightly and refrigerate. The tuna will keep, covered in oil and refrigerated, for up to 2 weeks.