If using sausage, brown sausage in the bottom of a dutch oven or large pot.
Add diced tomatoes, beef broth, water, pepper, Italian dressing mix, garlic and onion. Bring to a boil. Reduce heat and simmer for 5 minutes. Add leftover roast beef if using.
Bring to a boil, add tortellini and cook for 7 minutes (or per package directions). Sprinkle with pepper and serve!