In a medium saucepan, melt the butter. Add the onion and pepper and season with salt. Cook over moderate heat, stirring occasionally, until tender and golden, about 10 minutes. Add the flour and cook, stirring, until deep golden in color, about 5 minutes. Whisk in the port and stock and bring to a simmer. Cook over low heat, stirring occasionally, until thickened, about 10 minutes.