How to Make It
In a small bowl, stir the fish sauce with 2 1/2 tablespoons of the sugar, the water, lemon juice, lime juice, garlic, ginger and the sliced chile.
In another small bowl, toss the tomatoes with the soy sauce, vinegar and a pinch of sugar and let stand for 5 minutes.
In a medium bowl, gently toss the tuna and watermelon with the dressing. Using a slotted spoon, transfer the salad to 4 plates, allowing as much dressing as possible to drain back into the bowl. Using the slotted spoon, arrange the tomatoes on the plates. Garnish the salads with the tobiko and basil and serve right away.