Heat olive oil in a large saucepan, over medium heat. Add leek, carrots, zucchini, green beans and celery. Cover, and reduce heat to low. Cook for 15 minutes, shaking the pan occasionally.
Stir in the stock, tomatoes and thyme. Bring to a boil, then replace the lid, and reduce heat to low; simmer gently for 30 minutes.
Stir in the cannellini beans with liquid and tortellini. Simmer for an additional 15 to 20 minutes, or until pasta is al dente. Season with salt and pepper to taste before serving. Sprinkle esch serving with Parmesan cheese.