Place a large heavy bottomed pot over a medium heat and add the oil. When the oil gets hot, add in the curry leaves and onion along with a pinch of salt. Cook the onions for about 5 minutes, stirring often or until they turn light brown and caramelized.
Now add in the ginger garlic paste and tomato paste and cook this mixture for 2 minutes on medium heat or until it doesn't smell raw anymore. Deglaze the pan with ¼ cup of the chickpea cooking liquid/water if needed.
Add in the Kashmiri chili powder, coriander powder, cumin powder and turmeric powder & cook while stirring for another minute to let the spices bloom & release their aromatics.
Now add in the chickpeas & stir well so they are coated in all the beautiful flavors. Let the chickpeas toast in these flavors for about 2 minutes.
Pour in the can of coconut milk and mix everything until well combined. Let your curry bubble away gently for around 5 minutes.
Now add in the reserved chickpea water (about 1.5 cups total, add in more water if needed while canned chickpeas) along with the cubed sweet potatoes and let the curry bubble gently until the sweet potatoes are fully cooked (about 10 minutes).
Into a dry pan, add in the mustard seeds, fennel seeds and fenugreek seeds and toast them on medium heat for about a minute or until they just start to change color.
Transfer the toasted spices into a mortar and pestle and grind them into a fine powder.
When the sweet potatoes are cooked, add in the freshly ground spice mix and give it a mix before turning off the heat.
Serve the curry with my lemon rice or naan, side salad, a sprinkle of nigella seeds and cilantro and lime wedges on the side.