In a large skillet heat the oil over med-high heat and stir in the spices and onion. Cook about 3 minutes or until very fragrant and the onions have started to soften.
Add the remaining ingredients to the pan, stir to combine and bring to a boil, reduce heat and simmer for about 20 minutes or until the sweet potatoes are soft. Continue to simmer uncovered until the curry reaches the thickness that you'd like.