Combine shucked oysters and their liquor and 3 quarts cold water in a stock pot set over medium-high heat. Bring water to simmer until oyster edges have curled. Strain and reserve oyster stock and oysters separately.

In heavy bottom stock pot melt butter over medium-high heat. Add onion and celery. Saute until translucent, about 3 minutes, and add garlic. Sauté for 2 minutes and add flour. Stir flour until light roux forms. Add reserved oyster stock and milk. Stir till roux is dissolved. Add all remaining seasonings, except parsley.
Reduce heat to medium-low and simmer until thickened, 15 to 20 minutes. Add spinach and parsley and stir. When spinach has wilted add diced Brie. When Brie has melted add oysters simmer additional 5 minutes.
Serve.