Heat coconut oil in a large pot over medium heat. Add onion and cook until softened, about 5 minutes.
Add garlic, ginger, turmeric, cumin, and chili powder. Cook for 1 minute more, until fragrant.
Stir in red lentils, sweet potato, vegetable broth, and diced tomatoes. Bring to a boil, then reduce heat and simmer for 30 minutes, or until lentils and sweet potato are tender.
Stir in lemon juice and season with salt and pepper to taste.
Serve hot, garnished with fresh cilantro or a dollop of yogurt, if desired.
Notes
For a creamier dal, you can blend a portion of it with an immersion blender before serving.