Make the tuna confit: Dice the belly into large, similarly-sized chunks and cover in the kosher salt for 2 hours. This process cures the tuna, pulling out the moisture and seasoning it. Afterwards, rinse the tuna.
Warm the olive oil in a medium sauté pan or saucepan (whatever will hold and submerge the tuna) on low heat. Add crushed garlic, lemon zest, orange zest, and the herbs. Heat slowly until the garlic has cooked through and is slightly caramelized, about 20 minutes 3. Add the rinsed tuna to the olive oil and cook on low heat until tender. The cooking time will depend on the size of your pieces and can take anywhere from 25 to 40 minutes, but allow the tuna to cook low and slow so it can absorb the olive oil.
Let the tuna cool to room temperature while fully submerged in the olive oil before serving. The longer it sits, the more flavorful it will be.
Assemble the salad: Arrange the onions, olives and tomatoes on a plate. Flake the tuna over the salad, dress with sherry vinegar, the oil from the tuna, flaky salt and pepper and serve.