Heat a large soup pot or Dutch oven over medium heat. Add the Italian sausage and diced onion. Cook, breaking the sausage up with a wooden spoon, until no pink remains and the onion is soft and translucent, about 6 to 8 minutes. Drain excess fat.

In a small bowl, whisk together the water and cornstarch until fully dissolved. Pour into the pot along with the beef broth, diced tomatoes with juices, basil, and oregano. Stir well and cook for 5 minutes over medium heat.

Bring the soup to a full boil. Add the tortellini and sliced zucchini, then reduce the heat to a gentle simmer. Cook for 6 to 7 minutes or until the tortellini is tender and puffed. Season with salt and pepper to taste.

Ladle into bowls and top with shredded Parmesan cheese and fresh chopped parsley. Serve immediately.