If you are using shop-bought paneer, put the cubes in a heatproof bowl and cover with boiling water. Leave to soak for 10 minutes, then drain and pat dry with kitchen paper. (You don’t need to soak homemade paneer.)
Heat the oil in a kahari or wok (the oil is ready for frying when it reaches 180°C on a food thermometer, or when a cube of bread dropped into the oil browns in 30 seconds), and deep-fry the paneer in batches over a medium heat for about 2–3 minutes, until golden brown. (Alternatively, deep-fry in a large, wide, sturdy pan no more than two-thirds full with oil.) Drain the paneer cubes on kitchen paper and set aside.
Heat the 4 tablespoons of oil in a separate karahi or wok over a medium heat. Add the cumin seeds and fry, stirring all the time, for about 30 seconds, until the seeds darken and release a nutty aroma.
Stir in the onion and cook for about 7–10 minutes, until golden. Reduce the heat slightly, add the garlic and ginger, and fry for a further 2–3 minutes, until the garlic and ginger turn golden. Stir in the turmeric, chilli powder and garam masala, then add the chopped tomatoes, tomato puree and sugar. Cook over a medium heat for about 3–5 minutes, until the tomatoes have softened, adding a little water if they catch on the bottom of the pan. Reduce the heat to low and simmer, uncovered, for 7–10 minutes, until the masala has thickened.
Pour in 100ml of water and, as soon as the masala starts bubbling, add the peas. Cook for 3–4 minutes, until the peas are tender. Stir in the cubed paneer and chopped coriander and serve with chapatis.