Whisk together the ginger, lime juice, fish sauce, sugar, salt and pepper in a bowl until the sugar has dissolved. Slowly whisk in olive oil until incorporated. Set aside and refrigerate half of this mixture to be used as a salad dressing later. Whisk the garlic, lime zest, minced chilli pepper and coriander stems into the marinade. Coat the tuna steaks with the marinade, cover and let stand for 1 hour at room temperature.
Preheat a large, heavy-bottomed frypan over medium-high heat. Once the frypan is very hot, remove the tuna from the marinade and place into the frypan. Sear the tuna until golden brown on both sides, and cooked to your desired degree of doneness, about 2 minutes per side for medium, depending on thickness.
Place the watercress in a large bowl with the coriander leaves, cherry tomatoes and sliced chilli pepper. Pour in the reserved dressing and toss to mix. Serve the seared tuna on a bed of salad.