Heat four tablespoons of oil in a large, wide, nonstick pan. When very hot, add the aubergine and cook, stirring regularly, for 15 minutes, until soft and browning. Add half the minced garlic, cook for two minutes, then stir in the cinnamon, pepper, cumin, tomato puree, breadcrumbs and a teaspoon and a quarter of salt. Cook for a couple of minutes, then turn off the heat and leave to cool.
Once cool, blitz in a food processor and rinse out the pan. Using clean hands, roll the mixture into 16 koftas (about 35g each) and put on a plate.
To cook the koftas, heat two tablespoons of oil in the same nonstick pan. When hot, lay in eight koftas, spacing them well apart, and leave undisturbed for a few minutes, otherwise they’ll stick to the pan. When the bottom is crisp, use a fish slice to lever them off the pan, flip and cook until crisp and burnished mahogany all over – eight minutes in all. Transfer to a plate, repeat with the remaining eight koftas, then set aside while you make the sauce.
Heat three tablespoons of oil in the same pan over a low to medium heat and, once hot, add the rest of the minced garlic and cook for about three minutes, until sticky and golden. Add the passata and 200ml water, and leave to cook for 15 minutes, until jammy and thick. Add the Kashmiri chilli, garam masala and a teaspoon of salt, cook for five minutes more, then add the spinach handful by handful and stir in to wilt.
Turn down the heat to a whisper, pop the koftas into the sauce and leave to warm through gently. Serve with naan for dipping and scooping.