In a large pot over medium high heat add in oil and butter. Once butter begins to melt, add in clean leeks. Stirring occasionally, allow the leeks to cook for about 4 minutes, or until softened. Next, add in celery, carrots and seasonings. Add in vegetable broth and lower heat to a simmer, cook for 20-25 minutes or until carrots are tender.

While you're waiting, in a small bowl whisk together water and cornstarch, set aside. Once carrots are tender remove thyme and add in chicken and tortellini, raise heat to medium high heat, and cook until tortellini has softened, about 4-6 minutes. Next add in cornstarch slurry and stir gently until broth has thickened, about 2 minutes.

Turn off heat and allow soup to cool slightly before adding in half and half. Stir gently once and serve immediately.