Put the tomatoes, onion, cucumber and olives in a bowl. Tear the lettuce into pieces and add to the salad. Toss well and arrange on a large platter.
Whisk all the dressing ingredients together in a small bowl and season with salt and pepper. Drizzle a little over the salad.
Heat a heavy-based frying pan until hot, add the haloumi slices and cook for 1 minute on each side until charred and softened. Arrange on top of the salad, drizzle over the remaining dressing and serve immediately.