Drain oysters, reserving liquor and oysters separately.

Melt butter in a 4-quart saucepan over medium heat; add flour and whisk well. Cook, whisking constantly, for 5 minutes or until roux is golden. Add onion and celery; cook 5 minutes, stirring often. Add reserved oyster liquor and half and half. Cover, reduce heat to low, and simmer 20 minutes. Add reserved oysters and parsley. Season with salt and pepper; stir well. Simmer 1 minute or until the edges of the oysters begin to curl.

Remove from heat and serve immediately with hot buttered French bread.