Put the chicken stock and celery in a large saucepan, bring to a boil and simmer for about 3-4 minutes or until the celery is tender, but still has a little crispness to it. Remove from the heat, add the Parmigiano Reggiano, stir, cover and set aside. Cook the tortelloni according to the instructions on the pack and drain, reserving a large ladleful of the pasta cooking water. Add the pasta to the celery-parmesan mixture in the saucepan, along with the crème fraîche and parsley. Bring it all to a low boil, cook for 1 minute and season with salt and pepper to taste. Divide among plates and garnish with additional parsley if you like.