1. Preheat the oven to 325°F. 
  2. Add 1 tbsp of the oil to a heavy-bottomed medium saucepot over medium high heat. Toss the stewing beef with the !our and season with salt and pepper. Shake off any excess flour (but reserving it) and brown the meat for 2-3 minutes to a dark brown in 2 batches, adding the remaining 1 tbsp of oil for the second batch of beef. Set aside the browned beef while preparing the vegetables. 
  3. Turn the heat down to medium and add the onion, celery, carrots and mushrooms. Cook the vegetables for about 5 minutes, until the onions are translucent. Add any remaining flour from the beef and stir in for another minute. Add the prunes, beer and beef stock. Stir and bring to a simmer then reduce to a gentle simmer, cover and place in the oven for 90 minutes or until the meat is tender but not falling apart (alternatively, the stew can simmer gently on low heat for the same time, 90 minutes). 
  4. Immediately before serving, add the vinegar and brown sugar and season to taste.