Drain the chickpeas and rinse. Place 150g chickpeas in a blender or food processor with the tin of chopped tomatoes, 3 cm piece of chopped ginger, 4 cloves chopped garlic and the chopped chilli if using. Blend until well combined.
Heat the oil in a large saucepan with a lid. Add the cumin seeds once hot and turn the heat to medium.
Once the cumin seeds are starting to turn brown (about a minute) add the chopped onion and stir well. Turn the heat down and cook for 7-10 minutes until turning transparent. Stir often.
Add 1 tsp ground turmeric and stir. Cook for one more minute.
Add the 650g diced potatoes. Stir to coat in the onion mixture.
Next add the tomato mixture and 300ml water. Bring it the boil then reduce to a simmer. Cover with a lid and cook for 30-40 minutes until the potatoes are soft.
Stir occasionally and add a little more water if the sauce gets too thick.
Once the potatoes are soft add the rest of the chickpeas, 2 tbsp garam masala, 1 tbsp lemon juice and a pinch of salt (optional). Add a little water if needed and warm through.
Before serving add fresh coriander leaves.
Serve with rice or flatbreads and a yoghurt sauce.