2.In a medium saucepan, heat oil; cook onion, stirring, until soft. Add garlic; cook, stirring, until fragrant. Add sauce and basil; simmer, uncovered, 5 minutes. Stir in stock; season to taste.
3.Meanwhile, boil, steam or microwave potatoes until tender; drain. When cool enough to handle, slice thinly.
4.Heat oiled grill plate (or grill or barbecue); cook eggplant until browned lightly.
5.Spoon half the tomato mixture into 2-litre (8-cup) ovenproof dish; layer potato, zucchini, eggplant and fish in rows in dish. Pour remaining tomato mixture into dish. Bake, uncovered, about 15 minutes or until fish is cooked.