Scrape off the fresh tuna from the whole piece with a normal tablespoon so that you get a coarse tartare. Mix this well with lemon juice, a little olive oil, gin and 1 - 1.5 tablespoon soy sauce and, ideally, keep covered in a cool place.
For the pesto, roughly chop the coriander and garlic. Grate the parmesan, if it hasn`t been grated. Pour the pine nuts and salt into a tall vessel and grind with a hand blender. Fill up with olive oil until it has the consistency of toothpaste. Mix these in a ratio of about 1: 3 of pesto to tartare with the tuna tartare and season with soy sauce.
As a variation, you can also replace about a quarter of the coriander with Thai basil or refine the tartare with a little finely chopped ginger.
The rest of the pesto can be kept in a closed container in the refrigerator for at least a week and goes wonderfully with pasta or starters.