Preheat oven to 180°C (355°F). Place the tomato on a baking tray and brush with oil. Roast for 20 minutes or until cooked. Set aside. Brush the bun halves with oil and place on a baking tray. Grill (broil) under a preheated hot grill for 1 minute or until golden. Set aside. Heat a char-grill pan or barbecue over high heat. Brush the eggplant with oil and char-grill or barbecue for 2 minutes each side or until cooked through. Set aside. Brush the haloumi with oil and char-grill or barbecue for 1 minute each side or until golden. Spread the bun halves with harissa mayonnaise and top with the tomato, eggplant, haloumi, caramelised onions and rocket leaves to serve. Serves 4.