Grilled Tuna-and-Watercress Salad

Name: Grilled Tuna-and-Watercress Salad
Ingredients: 1 ½ cups flaked grilled tuna steak; 8 halved grape tomatoes; 2 boiled red potatoes, quartered; ½ small red onion, sliced; Oil and vinegar; 1 bunch watercress