Heat the oil in a large frying pan or wok over a high heat until visibly hot. Add the mustard seeds and when they begin to crackle (after about 30 seconds), reduce the heat to medium–high and toss in the cumin seeds and curry leaves.
Let these flavours infuse into the oil for about 30 seconds. Stir in the onion and fry for about 5 minutes until soft and translucent. Add the green chillies and garlic and ginger paste and stir it all up to combine.
Fry this base masala for about 1 minute, then stir in the ground spices, followed by the squash and fry for about 5 minutes, or until the squash is about half cooked through.
Add the coconut milk and the peas and cover the pan. Simmer for about 5 minutes until the squash is soft and the peas are cooked through.
You might need to add a little water, but remember that this is a dry curry, so don’t add too much unless you prefer a soupier curry, which is fine. Season with salt and garnish with the garam masala and coriander.