Step 1 Place a large saucepan over medium heat. Add olive oil; heat. Add chopped onion and celery; cook, stirring, for 3–4 minutes. Add eggplant and cook, stirring, for 2–3 minutes, or until browned. Add garlic and oregano; cook, stirring, for 1 more minute. Add beef mince and cook, breaking up mince with a wooden spoon, for 5 minutes, or until browned.
Step 2 Stir tomato paste into mince mixture; cook for 1 minute. Add tomatoes with ½ cup water and bring to the boil; simmer for 10 minutes, or until sauce thickens.
Step 3 Meanwhile, use a a vegetable peeler to slice long ribbons from zucchini, stopping at the seeds. Combine ricotta, egg and half of the parmesan in a medium bowl; whisk until smooth.
Step 4 Preheat oven to 160°C. Spray a 2-litre (8-cup) ovenproof dish witholive oil. Spread half of the mince mixture across base of dish, then top with an overlapping layer of half the zucchini ribbons; repeat with remaining mince mixture and zucchini ribbons. Spread ricotta mixture evenly over zucchini layer; scatter with remaining parmesan.
Step 5 Transfer dish to oven and bake for 20–25 minutes, or until cheese is bubbling and golden. Leave bake to cool for about 5 minutes, then serve with salad greens.