To the slow cooker, add the onion, garlic, ginger, lamb pieces, tomato paste, curry paste, tinned tomatoes, chicken stock and water. Stir well to combine. Place the lid on and cook for 3 hours on high or 7 hours on low.
Add the pumpkin and zucchini to the curry and stir through. Place the lid on and cook for another 1 hour on high or 2 hours on low.
Serve the curry on rice, top with a dollop of greek yoghurt and fresh coriander.