In a large pot, sauté onion with butter over medium heat. Add stock, celery, salt, pepper, bay leaf, and thyme. Bring mixture to a boil. Add haddock and simmer on low heat for 10 minutes. In a small bowl, mix 1 cup of the milk with flour. Add flour mixture to pot. Bring mixture to a gentle boil. Add remaining 2 cups of milk, heavy cream, crabmeat and clams. Turn off heat and let sit. Remove Bay Leaf before serving. Garnish with parsley. Serve with crusty bread.