We used a selection of green heirloom tomatoes for this salad, but it tastes just as good with other varieties. Be sure not to use unripe green tomatoes, though, which are best for frying or pickling.
Place onion in a small bowl of ice water. Let stand 10 minutes, then drain and pat dry. Meanwhile, whisk together orange zest and juice and vinegar. Whisk in olive oil until emulsified. Season with 1 teaspoon kosher salt and 1/4 teaspoon pepper.
In a large bowl, toss tomatoes, sliced fennel, olives, and drained onions with vinaigrette. Let stand at least 15 minutesand up to 2 hours.
When ready to serve, fold in chopped parsley. Season with kosher salt and pepper. With a slotted spoon, transfer salad to a serving plate or bowl. Spoon some of juices from bottom of bowl over top. Pour remaining juices into a small serving bowl or pitcher.
Toast bread, then drizzle with oil and rub with garlic. Break up tuna into large flakes and scatter over top of salad. Sprinkle with flaky salt, pepper, and fennel fronds. Serve salad with extra tomato juices and bread.