Trim fat from pork; discard. Cut pork crosswise into 1/4 inch thick slices. Stack a few slices at a time and cut 1/4 inch wide julienne strips. Cut onions in half and cut crosswise into slices. Cut pepper into 1/2 inch pieces. Finely chop garlic.Heat the wok over high heat about 1 minute or untl hot. Drizzle oil into the wok and heat 30 seconds. Add pork and stir fry until well browned. Reduce heat ot medium.Add onions and garlic to pork; stir fry 1 minute. Stir in tomato sauce, sugar, hoisin, vinegar, and salt. Cover the wok and cook 5 minutes, stirring occasionally. Stir in pepper and stir fry until crisp tender. Serve pork mixture over toasted buns, if desired.