Cut the meat into 1 1/2-inch cubes. Melt a tablespoon of lard in a casserole, fry the meat. If the lard browns, take it out with the meat, put some lard in again to fry the chopped onions without browning them. Put in the meat again, moisten with the beer and enough water to cover the meat. Salt, pepper. Spread the slice of bread with a level tablespoon mustard, add it with the bouquet garni. Cover. Bring to a boil quickly, then turn down the heat to cook gently for at leat 2 hours. Sieve the sauce, mashing the bread which must thicken it. Put 1 tablespoon of sugar into a saucepan, moisten with as much vinegar again, make a dark caramel. Dissolve in 2 tablespoons of boiling water, add to the sauce to intensify the color. Put all the ingredients together, adjust the seasoning, cook gently for a further 20 or 30 minutes before serving. Serve with boiled potatoes. If the sauce was too thin, before mixing in the caramel, add a large tablespoon of butter worked with the same amount of flour.
