Cut watermelon crosswise into 1 cm (1/2 inch) slices, then cut into wedges about 4 cm (1 1/2 inches) wide. Remove and discard any visible seeds.
Arrange watermelon wedges in a large shallow bowl. Scatter onion and all but a few mint leaves over the top. Put oil, vinegar and a good grind of pepper in a jug and whisk to combine. Drizzle over salad.
Crumble labna or goat's cheese over salad and season with pepper. Scatter remaining mint leaves over the top and serve.
This salad is best assembled close to serving time, although the individual components can be prepared 2 hours beforehand.