Heat the oil in a pan over medium heat until it is hot but not smoking.
Stir in the chopped onion and cook, stirring regularly, until softened and lightly golden, about 4–5 minutes.
Mix in the garlic paste and sauté briefly until the raw aroma disappears, around 1 minute.
Pour in the puréed tomatoes and cook them down, stirring often, until thicker and glossy, roughly 4–5 minutes.
Sprinkle in the chilli powder, garam masala, cumin powder, turmeric powder, and salt, then cook the spices gently for 3–4 minutes so they release their flavour.
Pour in a small amount of water and simmer, stirring occasionally, until the oil begins to separate from the masala.
Add in the potatoes, carrots, and peas, mixing well so everything is coated in the spiced base, then cook for 4–5 minutes.
Add the remaining water and stir until the curry is evenly combined.
Cover the pan and let it cook on medium heat until the vegetables are tender and the curry has thickened, about 15–20 minutes.
Finish by stirring in the coriander and mixing well before serving.
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