Heat oil in large pot. Add sausage, onion, garlic cloves, thyme and crushed red pepper flakes. Saute until vegetables are soft and sausage browned, about 12 minutes. Add broth and bring to a boil. Reduce heat to low and simmer , about 30 minutes. (If desired, can be prepared 1 day ahead to this point. Cool and refrigerate or freeze for longer storage.) Bring soup back to a simmer, if necessary, and add the cannellini beans and the tortellini to soup. Simmer until pasta is just barely tender, about 5 minutes. Finally, add the baby greens of choice (kale or spinach) and stir just until wilted. Ladle soup into bowls. Serve, passing grated cheese separately.
This was adapted from a recipe that appeared in the February 2002 issue of Bon Appetit magazine.