Heat the oil in a large pot over medium heat. Add the cumin seeds and cook for 30 seconds until they start to pop.
Stir in the onion, garlic, and ginger. Cook for 3 minutes until the onion softens.
Add the turmeric, coriander, garam masala, and chili powder. Stir for 1 minute to toast the spices.
Add the potatoes, turnips, and lentils to the pot. Pour in the vegetable broth and add the salt. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 30 minutes, stirring once or twice, until the lentils and vegetables are tender.
Remove the lid and simmer for 5 more minutes if you prefer a thicker consistency. Serve hot with chopped cilantro on top.