Heat the oil and butter in a large saucepan and add curry powder, turmeric, cumin seeds, mustard seeds, garam masala, chilli flakes, ginger, garlic, lemon zest, and honey. Cook for 2 minutes until fragrant.
Add the onion, kūmara, and carrot. Stir until well coated.
Add the lentils, tomatoes, and stock. Season generously, bring to a boil, then reduce heat and simmer for 30 minutes until vegetables are tender.
Lightly crush some of the vegetables to give a rough-textured soup. Adjust consistency with extra stock if desired.
Ladle into bowls, garnish as preferred, and serve.
