In a large, heavy-bottomed stock pot, brown the sausage over medium heat, breaking apart as it cooks. Drain the excess grease -- optional, but I prefer a less greasy broth.
Add the Onion & Garlic
Finely chop the onion and garlic and stir them into the cooking sausage. Cook 3-4 minutes, stirring occasinoally, until fragrant.
Add the Broth & Tomatoes
Pour the broth and tomatoes, stir, and bring to a simmer.
Add the Tortellini
Stir in the tortellini and cook together for about 3-4 minutes (fresh tortellini cooks very quickly, but if you're using frozen, just cook according to the directions on the package).
Add the Spinach
Just before you're ready to serve, stir in the baby spinach and cook together just until it's gently wilted. You can do this one or two handfuls at a time, if that fits better in your pot.
Serve
Season to taste with salt, pepper and crushed red pepper, if using. Divide the soup into bowls and sprinkle with fresh parsley.