Spoon 1-inch chunks of sausage into a 4- to 6- quart non-stick pot; cook sausage and ground fennel over medium-high heat, stirring frequently, until meat is lightly browned and no longer pink. Stir in stock, tomato sauce and diced tomatoes. Bring to a boil, reduce heat to low; cover and simmer 10 minutes. Increase heat to high, stir in tortellini and squash. When liquid returns to a boil, stir once, add basil, if desired; reduce heat to medium-low. Cover and cook until pasta is just tender, about 6 to 8 minutes. Season to taste; garnish with Parmesan and parsley.