Heat oil in stockpot over medium heat until hot. Place beef Chuck Shoulder Pot Roast in stockpot; brown evenly. Remove pot roast from stockpot. Pour off all but 1 teaspoon drippings.
Add onion to stockpot; cook and stir 3 to 5 minutes or until crisp-tender. Return roast to stockpot. Stir in salsa; bring to a boil. Reduce heat; cover tightly and simmer 2 to 3 hours or until roast is fork-tender.
Remove roast; keep warm. Skim fat from cooking liquid; keep warm. Carve roast into thin slices; stir into cooking liquid. Season with salt and pepper, as desired.
Warm rolls, if desired. Layer cheese and beef mixture evenly onto bottoms of rolls. Close sandwiches. Serve with additional cooking liquid.
Nutrition information per serving: 728 Calories; 342 kcal Calories from fat; 38 g Total Fat; 15 g Saturated Fat; 11 g Monounsaturated Fat; 141 mg Cholesterol; 971 mg Sodium; 49 g Total Carbohydrate; 2.7 g Dietary Fiber; 48 g Protein; 5.1 mg Iron; 7.4 mg NE Niacin; 0.4 mg Vitamin B6; 2.3 mcg Vitamin B12; 7.9 mg Zinc; 56.8 mcg Selenium;