Put the onion in a small bowl and pour over the lime juice. Set aside.
Toast the coriander seeds in a dry frying pan until fragrant. Transfer to a mortar and lightly crush. Mix with the salt, black pepper, cayenne pepper and cumin.
Gently combine the watermelon, cucumber, marinated onion and chopped coriander in a large bowl. Sprinkle over the spice mix and toss. Taste for seaso