Stir in the turmeric powder, cumin powder, coriander powder, and chili powder (if using). Cook for about 30 seconds, stirring constantly, until the spices are fragrant.
Add the rinsed red lentils and vegetable broth to the pot. Bring to a boil, then reduce heat and simmer for about 20-25 minutes, or until the lentils are tender.
Stir in the chopped spinach and cook until it wilts, about 2-3 minutes.
Stir in the lemon juice and season with salt and pepper to taste. Serve hot.
Notes
Serve with rice or naan bread for a complete meal.