1. Heat oil in stockpot over medium heat until hot. Place beef pot roast in stockpot; brown evenly. Remove pot roast from stockpot. Pour off all but 1 teaspoon drippings.
2. Add onion to stockpot; cook and stir 3 to 5 minutes or until crisp-tender. Return pot roast to stockpot. Stir in salsa; bring to a boil. Reduce heat; cover tightly and simmer 2 to 3 hours or until pot roast is fork-tender.
3. Remove pot roast; keep warm. Skim fat from cooking liquid; keep warm. Carve pot roast into thin slices; stir into cooking liquid. Season with salt and pepper, as desired.
4. Warm rolls, if desired. Layer cheese and beef mixture evenly onto bottoms of rolls. Close sandwiches. Serve with additional cooking liquid.