Melt the butter in a large skillet. Add the onion and cook over low heat until softened, about 7 minutes. Stir in the egg and cook, stirring, until dry and crumbly, about 30 seconds. Add the manioc flour and cook over moderate heat, stirring occasionally, until lightly toasted, about 8 minutes. Stir in the parsley and season with salt. Serve with the feijoada.