Heat oil in a large pot over medium heat. Saute onion till its turning translucent an a little golden. Add the lentils, potato, carrots, and spices/seasonings, and stir to combine everything.
Add the broth or water to the pot and bring to a boil. Reduce to a simmer and cook for 25 minutes, or until the lentils and sweet potato are tender.
Allow lentils to cool a bit, then serve with fresh avocado slices.