Heat the oil in a large pot over medium heat. Saute the onion until it's turning a little golden. Add the lentils, potato, carrot, and spices/seasonings, and stir to combine everything.
Add the water or broth and bring to a boil. Reduce the heat to a simmer and cook for 25 minutes, or until the lentils and sweet potato are tender. Stir in the 1/2 cup of coconut milk. Serve.
Curry will keep for up to 4 days in the fridge. Leftovers can be frozen.