Slice meat into 1/2x2x21/2 inch rectangles, season with salt and pepper. Tie the herbs of the bouquet garni together. Spread mustard on bread.
Preheat oven 350F. (180C.) Heat oil and quickly brown meat over high heat, set aside. Melt butter in saute pan add onions, cook until golden brown. Add the brown sugar cook until lightly carmelized, then pour in vinegar and evaporate it over high heat.
Arrange alternate layers of meat and onions in ovenproof 4qt casserole, inserting the slice of bread and the bouquet garni in the middle. Pour the beer in a saute pan and heat it over high heat. Transfer it to casserole adding more to cover meat. Cover and braise in oven for 4 hours without disturbing.
Transfer the meat to a shallow dish and keep warm. Puree the bread, onions and cooking juices through the fine holes of a food mill or food processor to make a smooth sauce. Reheat the sauce over high heat, then pour over the meat. Serve immediately.
About this recipe:
In Flandres most meats are served with fried potatoes, and carbonnades are no exception, but you could also serve them with steamed or mashed potatoes. This dish is an everyday dish both in Belgium and France.