Heat oil in a frying pan, add onion and garlic. Cook for 2-3 minutes until tender. Stir in spices and cook for a further minute. Transfer onion mixture to the pot of the slow cooker with the tomatoes, stock, water, paste, potatoes and lentils. Stir to combine and cook on low for 2 hours.
Add corn and zucchini to pot, mix to combine and cook for a further 2 hours on low. Season to taste and serve with rice. Accompany with yoghurt, coriander sprigs and lemon wedges.