Arrange the cucumbers, tomatoes, onions and peppers, artichoke hearts and parsley on a platter. Squeeze the lemon juice over the platter, juicing the fruit cut-side-up to keep the seeds from falling into salad. Season with salt and pepper.
Place shallots, vinegar and oregano in a bowl and let stand 5 minutes. Whisk in extra-virgin olive oil in a slow stream. Pour dressing over salad. Garnish salad with chunks of tuna, hot peppers and olives.