1. Peel and cube the potatoes.
2. Thinly slice the red onion.
3. Measure out all spices and have them ready.

1. Heat the vegetable oil in a large frying pan over medium-high heat.
2. Add the mustard seeds and toast until they start to pop.
3. Stir in the garam masala, turmeric, cumin, coriander, and chilli flakes, cooking for about 30 seconds until fragrant.
4. Add the sliced onion and sauté for 5 minutes until softened.
5. Transfer the spiced onion mixture to a slow cooker.
6. Add the cubed potatoes and chopped tomatoes, seasoning with salt and pepper.
7. Cook on high for 4 hours or on low for 6 hours, until the potatoes are tender but still hold their shape.

Serve the potatoes hot, garnished with roughly chopped coriander leaves.