To make the dressing, place the oil, vinegar, mustard, tarragon, salt and pepper in a bowl and mix well to combine.
Place the zucchini, salt and pepper in a bowl and gently toss to coat. Heat the oil in a large non-stick frying pan over medium heat. Cook the zucchini for 1−2 minutes or until tender. Set aside.
Brush the fish with oil, salt and pepper and cook, skin-side down, for 3 minutes. Turn over and cook for a further 3 minutes or until cooked through. Place the zucchini, spinach, basil and dressing in a bowl and toss to combine. Arrange the zucchini and spinach salad and serve with the fish. Serves 4.