Heat the oil in a large frying pan or wok over high heat. When visibly hot, add the mustard seeds. After a few seconds, they will begin to pop in the oil. Lower 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 chopped onion and fry for about five minutes until soft and translucent. Then add the green chillis and garlic and ginger paste and stir it all up to combine.
Fry this base masala for about a minute and then stir in the ground spices, followed by the cubed squash and fry for about five minutes or until the squash is about 50% cooked through.
Add the coconut milk and bring to a simmer.
Then add peas to simmer in the sauce. Simmer until the squash is soft and the peas are cooked through.
To finish, season with salt to taste and garnish with the garam masala and chopped coriander.