Place watermelon, mixed salad leaves, basil and chervil in a large bowl and set aside.
Heat a grill pan to high. Grill tomatoes for 2 mins and add to salad. In a medium bowl, coat haloumi with olive oil. Grill haloumi for 2-3 mins each side, or until grill marks form. Once cooked, place in a small bowl and add lemon juice.
In a small bowl mix dressing ingredients together. Add haloumi to salad and pour over dressing.