In a large, deep skillet, heat the oil until shimmering. Add the onion, garlic and ginger and cook over moderately high heat until barely softened, about 4 minutes. Add the lamb and cook over moderately high heat, breaking it up with a wooden spoon, until it starts to brown, about 10 minutes. Add the curry powder and sweet potato and cook for 2 minutes. Add the coconut milk and stock and season with salt and pepper. Cover partially and simmer over moderate heat until the sweet potato is tender, about 15 minutes. Add the peas and cook until heated through. Stir in the cilantro and serve with hot sauce.