Heat the coconut oil in a large pot or Dutch oven over medium heat.
Add the diced onion and sauté for 5 to 7 minutes until translucent and softened.
Stir in the minced garlic, ginger, curry powder, turmeric, and chili flakes. Cook for 1 minute, stirring constantly, until highly fragrant.
Add the sweet potato cubes, rinsed red lentils, diced tomatoes, coconut milk, and vegetable broth. Stir well to combine.
Bring the mixture to a boil, then immediately reduce the heat to medium-low. Cover the pot and simmer for 20 minutes, stirring occasionally to prevent sticking, until the lentils have broken down into a creamy sauce and the sweet potatoes are fork-tender.
Remove the pot from the heat. Stir in the fresh lemon juice, then taste and season with salt and pepper as needed.
Ladle the dahl into bowls and serve hot, garnished with freshly chopped cilantro.