In a bowl, combine sausage and spinach and mix well. Scoop out 1 tsp. at a time and form into mini meatballs.
In a large saucepan, heat olive oil over medium-high heat, add meatballs, and cook until browned on all sides.
Lower heat to medium and add garlic, leek, and celery. Cook until soft, about 5 minutes. Add broth, squash, and salt. Bring to a simmer and cook 5 minutes. Add pasta and cook until both it and the squash are tender, about 7 minutes more.
To make the croutons, heat the broiler, butter bread slices, and use cookie cutters to cut shapes. Sprinkle with cheese, then place on a baking sheet. Broil for about 30 seconds, or until golden. Serve on soup.