In a large saucepan, saute the garlic and the onion in the butter until onion is translucent.

Add the broth, potatoes, carrots, celery [no tops], and zucchini to the pan.

Bring the soup to a boil, simmer for 15 to 20 minutes, or until potatoes are tender.

Add the dill, parsley, salt and pepper, reserved celery tops, and if desired, the thickener (1 T. cornstarch dissolved in 3T. water). Simmer the soup until it is slightly thickened. Serve immediately, with croutons, if desired.