Combine the mint, basil, tuna and lemon zest in a large bowl and stir gently. Taste and season with sea salt and freshly ground pepper. Add the extra-virgin olive oil, if using.
Spoon generous portions of tuna onto the crackers or endive pieces. Garnish with lemon wedges, mint and basil sprigs. Serve immediately.
For entertaining, the tuna can be diced in advance and kept well chilled in the refrigerator, then dressed when it's time to serve.