For the bacon sauce, fry the finely diced bacon in hot fat (or oil) until brown. Add the finely chopped onion and fry until light yellow.
Dust with flour, mix well, deglaze with a little cold soup, stir until smooth and pour in the rest of the soup.
Add the spice bag and cook slowly over low heat for 20 minutes. Remove the spice bag.
Season the bacon sauce with salt and pepper and add a splash of vinegar to taste.