In a large pot, heat a splash of water or a small amount of olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
Add the minced garlic and ginger to the pot and cook for another minute, stirring frequently to avoid burning.
Stir in the curry powder, turmeric, and cumin, cooking for an additional minute until the spices are fragrant.
Add the diced sweet potatoes, rinsed lentils, coconut milk, and vegetable broth to the pot. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and let simmer for 25-30 minutes or until the sweet potatoes and lentils are tender. Stir occasionally to prevent sticking.
Season with salt and pepper to taste. If desired, use an immersion blender to blend part of the curry for a creamier texture, then stir well.
Serve hot, garnished with fresh cilantro.