Brown Italian sausage in a skillet over medium heat, breaking it into crumbles, until no pink remains and the pieces have some color, about 8 to 10 minutes. Drain the grease.
Add drained sausage to the slow cooker.
Pour in beef broth, undiluted French onion soup, Rotel, diced tomatoes, water, and crushed basil. Stir to combine.
Cover and cook on Low for 8 to 10 hours.
About 30 minutes before serving, stir in frozen cheese tortellini directly from the freezer.
Cover and cook on Low for 25 to 30 minutes, until tortellini is tender but not mushy.
Stir, taste, and add a splash of broth or water if the soup has thickened too much.
Ladle into bowls and serve hot, with grated Parmesan or crusty bread if desired.
Notes
Do not add the tortellini at the beginning or it will get mushy. Use the condensed French onion soup undiluted for the best flavor. The soup thickens overnight because the tortellini absorbs broth; add broth or water when reheating. Freeze the soup base without tortellini for the best texture.
Keyword italian sausage tortellini soup, slow cooker soup