Wash clams thoroughly.

Steam clams in a 12-quart heavy stockpot until they open.

Reserve the broth from steaming the clams.

Remove clams from their shells and chop coarsely.

Cover and set aside both clams and broth.

In the same stockpot, saute crushed garlic in butter for about 3 minutes.

Add minced onion, minced celery, white pepper, bay leaf, and thyme.

Saute until onions are translucent.

Add flour to the pot to create a roux, stirring constantly.

Cook the roux over low heat for 5 minutes, stirring continuously.

Slowly add the reserved clam nectar and enough milk to reach 5 1/2 cups total, stirring constantly to avoid lumps.

Simmer for 10 minutes, being careful not to burn the soup.

Add diced potato and cook until tender.

Add chopped clams and heavy cream.

Bring the chowder back to a gentle boil.

Serve hot.