Put the lemon zest and juice into a screw-top jar. Press the garlic and salt together with the side of a knife until the garlic is mashed; scrape it into the jar. Add pepper to taste and the olive oil. Screw on the lid and shake well until blended.
Put the tomatoes, olives and onion in a large salad bowl. Shake the dressing again and stir about half into the salad. Add the greens and toss. Gently stir in the tuna, then taste the salad and add a bit more dressing if needed.