Put the onion, garlic, ginger, chilli and 3 tbsp of water into a blender or food processor, and whizz until completely smooth.
Heat the vegetable oil in a large, deep, non-stick frying pan, carefully add the onion purée and cook for 10 minutes until reduced and golden. Add the spices and cook for 2 minutes, then tip in the chopped tomatoes and bring to a simmer.
Season and add the potatoes and chickpeas along with 1⁄2 a tin of water and simmer for 30 minutes until thickened and the potatoes have cooked through. Add the spinach and cook for a few minutes to wilt.
Stir in the lemon juice and garam masala, and serve with naans.