Preheat the oven to gas 6, 200°C, fan180°C. Heat the olive oil in a wide medium saucepan over a medium heat. Add the chopped onion, season well and fry until softened. Add the finely chopped garlic, grated ginger, chillies, cumin, coriander and turmeric and fry for 2-3 minutes.
Add the mince, turn up the heat and fry for 10 minutes until golden. Drain off any excess fat. Add the stock and vine tomatoes to the saucepan, bring to the boil, cover and simmer for 20 minutes until the beef is cooked and the sauce has thickened slightly. Stir in the spinach, lemon juice and garam masala, and season to taste.
Put the potato and onion into a pan of boiling salted water and blanch for 30 seconds. Drain, then tip into a clean tea towel and squeeze out all the excess liquid. Put in a bowl, toss with the remaining olive oil and season. Tip the keema mix into a medium ovenproof dish and top with the blanched rosti topping.
Bake in the preheated oven for 25 minutes or until bubbling and the topping is crispy and golden. Serve with warm naans, if you like.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.