Put the vegetable oil in a large non-stick frying pan set over a medium-high heat. When hot, add the cumin seeds and fry until they turn dark brown and release their aroma.
Add the green chilli and diced white onions and fry for 3 minutes, then add the garlic and ginger and fry until the onions are translucent.
Add the ground turmeric, chilli powder, garam masala, ground coriander, ground fenugreek and tomato purée and stir until everything is fully combined.
Add the chopped tomatoes, 200 ml/7 fl oz/scant 1 cup water, salt and sugar, bring up the the boil, then reduce the heat to low and continue to cook gently for 15–20 minutes, stirring occasionally, until you have a thick and creamy sauce.
Add the frozen peas and spinach and cook for 2 minutes until the spinach has wilted, then add the paneer and stir well. Continue to cook for 3–5 minutes until the paneer is soft and giving, but no longer as it will become dense and tough.
Finish with the fresh coriander and green chilli just before serving.