Preheat oven to 350ºF. In a skillet over medium heat, add oil and when it gets hot, add onion; sauté until soft, about 2 minutes. Add beef and cook until no longer pink. Half way through cooking, crack some freshly ground black pepper.
When cooked, drain fat and return meat to skillet. Add the remaining ingredients except dinner rolls; mix well. Warm the mixture for 5 minute.
Make a small incision in the buns and stuff with beef mixture. Place them in a baking dish side by side facing up and bake for 10 minutes. Serve immediately with mustard or any preferred condiments.