Heat the vegetable oil in a large saucepan over a medium heat and add the cumin seeds, cooking them for about 1 minute until they start to pop. Add the onion and sweat for 4-5 minutes until soft, stirring occasionally.
Add the garlic and continue to cook, stirring occasionally, for 2 minutes. Add the ground cumin, ground cinnamon and chilli powder and cook for a further 1 minute. Stir in the cubed potato and add 250ml of water and the tomato. Cover and reduce to a simmer.
Simmer for 15-20 minutes until the potato is soft. Remove the cover, adjust the seasoning to taste then add the peas and cook for a few minutes until warmed through. Spoon into a serving bowl and serve immediately.