Rinse the red lentils under cold water until the water runs clear. Set aside.
In a large pot or deep skillet, heat coconut oil over medium heat. Add chopped onions and sauté until translucent. Add minced garlic and grated ginger, cooking for an additional 1-2 minutes until fragrant.
Stir in the curry powder, ground cumin, ground coriander, turmeric, and chili powder. Cook the spices for 1-2 minutes to release their flavors.
Add the diced sweet potatoes and rinsed lentils to the pot and stir until everything is coated in in the curry spices.
Now, add the diced tomatoes (with their juices), coconut milk, and vegetable broth to the pot. Stir well to combine.
Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for about 20-25 minutes, or until the lentils and sweet potatoes are tender.
Season the curry with salt and pepper to taste. Adjust the spice levels if needed by adding more chili powder.
Once the lentils and sweet potatoes are fully cooked and the curry has thickened, remove the pot from heat. Squeeze the juice of half a lime into the pot and stir to combine. Garnish with fresh cilantro if desired.
Serve the sweet potato lentil curry over cooked rice or with naan bread.