2) Spoon tuna mixture onto the bread slices and place on a foil-lined baking tray. Top the tuna with slices of cheese. Grill for 2 minutes, until the cheese melts.
For the salad:
1) In a small bowl, whisk together the oil, vinegar and mustard and season to taste with salt and black pepper.
2) Arrange the lettuce on a serving plate and top with grapefruit segments and walnuts. Pour the dressing over the salad and serve.