Heat the oil in a large pot over medium heat. Add the cumin seeds and cook until they begin to pop.
Stir in the onion, garlic, and ginger. Cook for 3 to 4 minutes until the onion softens.
Add the turmeric and red chili powder. Stir for 30 seconds to toast the spices.
Pour in the vegetable broth and add the rinsed lentils. Bring to a boil, then reduce to a simmer and cook for 20 minutes until the lentils are soft.
Stir in the chopped mustard greens and cook for another 5 minutes until they wilt but stay bright green.
Season with salt. Ladle into bowls and top with cilantro. Serve with lemon wedges on the side.