Heat oil in a large, deep frying pan over medium-high heat. Add lamb and cook, turning, for 10 minutes or until browned. Transfer shanks to a 5.5L slow cooker.
Add onion to frying pan and cook for 6 minutes or until softened. Add garlic and ginger and cook for a further 1 minute or until fragrant. Add curry paste and cook, stirring, for 2 minutes or until fragrant. Add tomatoes, stock and cardamom, then bring mixture to the boil, stirring constantly. Pour sauce over shanks. Cover with lid, then cook on high for 6 hours or until shanks are tender.
Combine yoghurt, mint and lemon juice in a small bowl. Season.
Transfer shanks to a dish. Remove and discard cardamom from sauce. Spoon sauce over shanks. Top with cucumber and extra mint. Serve with yoghurt mixture and pappadums.