In a medium sized pot, heat the oil. Then, add onion, celery, carrot and sweet potato. Cook until softened, about 6 to 7 minutes. Add the garlic and kale and continue cooking until kale wilts.
Add the lentils, bay leaf, rosemary sprigs, serrano pepper, stock and bring to a boil. Reduce heat and simmer until lentils are tender and most of the liquid is absorbed, about 30 minutes.
Season with salt to taste and remove the bay leaf, serrano pepper and rosemary sprigs before serving. Enjoy with a piece of crusty bread.