Heat the olive oil in a large pot over medium heat. Add the cumin seeds and let them sizzle for 30 seconds.
Stir in the diced onion and cook until it softens, about 5 minutes. Add the garlic and ginger and cook for another minute.
Add the turmeric, garam masala, and chili powder. Stir for 30 seconds until the spices smell fragrant.
Pour in the diced tomatoes with their juice and cook for 2 minutes, scraping the bottom of the pot.
Add the rinsed red lentils and vegetable broth. Bring to a boil, then reduce to a simmer. Cook uncovered for 20 minutes, stirring occasionally.
Stir in the salt and fresh spinach. Cook for 3 more minutes until the spinach wilts.
Serve hot with chopped cilantro and lemon wedges on the side.