Combine the oil and the flour in a large, heavy pot over medium heat. Stirring slowly and constantly, make a light brown roux. Add the onions and cook, stirring, until soft, 3 to 4 minutes. Combine the water (or milk) with the reserved oyster liquor and add slowly to the roux mixture, stirring constantly. The mixture will thicken slightly. Bring to a gentle boil, then reduce the heat to medium-low and simmer for 2 minutes. Add the oysters, parsley, and butter and simmer until the edges of the oysters curl. Remove from the heat.
Serve warm with crackers or hot French bread.
Yield: 6 to 8 servings