Step 1 Spray a large saucepan with oil and place over high heat. Cook lamb, in batches, for 2-3 minutes until browned. Transfer to a plate.
Step 2 Return pan to medium heat and spray with a little more oil. Add leek, carrot and celery and cook, stirring, for 5 minutes or until softened. Add stock, 2 cups water, tomato paste and rosemary. Bring to the boil. Reduce heat and simmer, covered, for 3 hours or until lamb is very tender. You can cook it for 4-5 hours if you like. Using a shallow spoon, skim off any visible fat.
Step 3 Add chickpeas and zucchini and cook for 15 minutes, or until zucchini is tender and chickpeas are heated through.
Step 4 Combine parsley, lemon zest and chopped garlic in a small bowl. Sprinkle over casserole. Serve with mashed sweet potato.