Heat the oil in a medium pot over medium heat. Add the shallot, garlic, crushed red pepper, and last tablespoon of curry powder, and stir until fragrant, about 10-15 seconds. Add the chicken and cook until the edges begin to turn golden, 3-4 minutes. Stir in the fish sauce, sugar, lemongrass, ginger, and chicken broth. Bring to a boil, then reduce heat to low, and add the carrots. After about 10 minutes, pour in the coconut milk. Fold in the onions and sweet potato, and simmer for 15-18 minutes, until the vegetables are tender.