2) Whilst onion is soaking, put the lemon zest, juice, salt, and pepper in a large bowl. Gradually whisk in the olive oil, starting with a few drops and then adding the rest in a steady stream to make a smooth, slightly thick vinaigrette.
3) Add the radishes, celery, cucumber, tomato, herbs and greens to the bowl. Drain the onion and add to the salad. Add the tuna and toss again, keeping the tuna in nice even pieces. Divide between two plates and serve.