Combine broth, garlic, rosemary and crushed red pepper in a 4- to 6-quart Dutch oven or soup pot; bring to a simmer. Partially cover and simmer over medium-low heat for 20 minutes to intensify flavor. Meanwhile, mash 1 cup beans in a small bowl.
Scoop garlic cloves and rosemary from the broth with a slotted spoon (or pass the soup through a strainer and return to the pot). Add mashed and whole beans to the broth, along with tomatoes; return to a simmer. Stir in pasta, cover and cook over medium heat, stirring occasionally, until the pasta is just tender, 10 to 12 minutes.
Stir in spinach, cover and cook just until the spinach has thawed, 2 to 3 minutes. Ladle the soup into bowls and garnish each serving with a drizzle of oil, if desired, and a sprinkling of Parmesan. Variation: Substitute chickpeas (garbanzo beans) for the cannellini beans; use a food processor to puree them.
Ingredient note: Individually quick-frozen (IQF) spinach is sold in convenient plastic bags. If you have a 10-ounce box of spinach on hand, use just over half of it and cook according to package directions before adding to the soup in Step 3.
