In a large Dutch oven set over medium/high heat, place in the coconut oil, onion, lemongrass, curry powder, sugar, and seasoning salt. Stir together and cook until the onions are translucent, about 3 minutes. Add in the raw chicken thigh, stir and continue to cook all the way through, about 5-7 minutes. Add in the sweet potato, coconut milk, and chicken broth. Bring to a boil.