In a large saucepan heat oil over medium heat, add leeks, onions and garlic and saute until onions are tender. Add reserved oyster liquid and cook 30 seconds. Add heavy cream and potatoes; season with salt, white pepper, 20 turns of black pepper, hot sauce and Worcestershire sauce; cook 4 minutes. Remove from heat and use an immersion blender to puree soup in saucepan.
Return saucepan to heat and add green onions, parsley and oysters. Cook just until edges of oysters begin to curl, about 2 minutes; be careful that oysters don't overcook. Remove from heat and add butter, stirring until melted. Ladle stew into 4 warmed shallow soup bowls, grind some pepper over and top with some croutons.