Heat coconut oil in a saucepan then cook mustard and cumin seeds until fragrant. Add onions to the pan and let caramelise.
Add garlic, ginger and curry leaves to the pan, and cook for 3-4 minutes. Mix in tomatoes, cashews, turmeric and chilli powder, then add lentils, split peas, water and salt. Stir until combined.
Bring to the boil and reduce to simmer for approx. 30 minutes or until lentils and split peas have softened. Stir occasionally to avoid it catching on the bottom of the saucepan. Mix through garam masala and cook a further 5-10 minutes.
Remove from heat and use a hand blender to blitz the tomatoes and cashews.