Make the dressing: Stir together all the ingredients except for the oil until well combined. Then whisk in the oil slowly to emulsify.
In a large bowl, toss together the onion, cucumber, tomatoes and olives. In a separate small bowl, toss the halloumi cubes with salt and pepper to season them.
Heat the oil in a large frying pan and fry the halloumi cubes over medium heat for about 2 minutes on each side, until golden and slightly soft inside. Remove from pan and set aside on a paper towel-lined plate.
Toss the salad vegetables with the dressing, turning them over with your hands to make sure everything is lightly coated. Add the halloumi cubes to the salad and toss gently to combine.
Divide the salad among 2 plates and serve immediately. Serves 2.