Preheat the oven to 425º F.
Place the cubes of bread into a large bowl and drizzle with about 2 tablespoons of extra virgin olive oil. Add a pinch of salt and a few cracks of black pepper, and toss to coat.
Transfer the bread to a rimmed baking sheet, spreading the cubes out evenly, and bake for about 10 minutes or until the bread is a golden crisp. Remove the bread from the oven and let it cool.
In the meantime, place the drained tuna, cucumbers, tomatoes, and onion into a large mixing bowl; set aside. In a small bowl, combine the garlic paste, red wine vinegar, fresh oregano, salt, chili flakes, Dijon mustard, capers, and caper brine. Whisk well with a fork and slowly drizzle in the remaining extra virgin olive oil, whisking until well blended.
Once the bread has cooled, add it to the tuna, cucumbers and tomatoes, then drizzle with the dressing, gently tossing it to coat everything evenly.
Let the salad sit for at least 30 minutes, and up to 4 hours, before serving. Prior to serving, toss the salad with a little extra olive oil, red wine vinegar, and salt if necessary. Garnish with fresh basil and parsley.