Heat the olive oil in a large pot over medium heat. Add the diced onion and cook for 5 minutes until it softens.
Stir in the minced garlic and cook for 1 minute until fragrant.
Add the cubed sweet potatoes and rinsed lentils to the pot. Stir to coat them with the oil and aromatics.
Mix in the drained chickpeas, harissa paste, and vegetable broth. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 25 minutes until the lentils and sweet potatoes are tender.
Season with salt and black pepper. Stir well and remove from heat.
Ladle the stew into bowls and top each serving with chopped cilantro.