Heat the oil in a large pot over medium heat. Add the chopped onion and sauté until translucent.
Stir in the minced garlic and grated ginger, and cook for another minute. Add the turmeric, cumin, and coriander, stirring well for another minute.
Mix in the rinsed red lentils, diced tomatoes, and vegetable broth. Season with salt and pepper.
Bring the mixture to a boil, then reduce the heat and simmer for 20-25 minutes, or until the lentils are tender.
If desired, add chili powder for heat and/or coconut milk for creaminess, and cook for an additional 5 minutes.
Garnish with fresh cilantro before serving.