Blanch salt pork in boiling water 5 minutes
Drain and rinse well; pat dry
Transfer to heavy large skillet and cook over medium flame until lightly browned
Remove from skillet using slotted spoon and reserve for use in salads or other dishes
Increase heat to medium high
Add beef to skillet in batches (do not crowd) and cook until well browned on all sides, turning with spatula
Transfer meat to heavy 2 to 3 quart saucepan
Add onions to skillet and reduce heat slightly and cook until deep golden brown, about 10 minutes, stirring occasionally
Blend in flour and cook about 30 seconds, watching carefully so flour does not burn
Add stout and stir, scraping up any browned bits
Bring mixture to boil
Pour over beef
Blend in vinegar, sugar, parsley, bay leaf and thyme
Cover saucepan and simmer mixture for 30 minutes
Spread mustard over bread
Press bread into stew
Cover and cook until meat is tender, about 1 hour
Remove bay leaf and discard
Serve immediately