If using sausage, brown sausage in the bottom of a dutch oven or large pot.
Add diced tomatoes, beef broth, water, red pepper, Italian dressing mix, garlic and onion flakes. 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.
Notes
Keep leftover soup in an airtight container in the refrigerator for 4 days and in the freezer for 1 month.