2tablespoons vegetable oil
4leeks, white and pale-green parts only, thinly sliced
2garlic cloves, finely choppedosher salt, freshly ground pepper
1cup dry white wine
2dozen oysters, freshly shucked, liquor reserved
4cups low-sodium vegetable brothPinch of saffron threads (optional)
1large bunch watercress, tough ends trimmed, chopped (about 3 cups), plus finely chopped for serving
1/2 teaspoon crushed red pepper flakes
1/2 cup heavy cream, plus more for servingCrusty bread (for serving)
Heat oil in a large heavy pot over medium-high heat. Cook leeks, stirring often, until softened, about 4 minutes. Add garlic and cook, stirring, until fragrant, about 1 minute; season with salt and pepper.
Add wine and reserved oyster liquor to pot and cook until slightly reduced, about 4 minutes. Add broth and saffron; bring to a boil, reduce heat, and simmer 5 minutes. Add watercress and cook until just wilted, about 2 minutes. Let cool slightly, then, working in batches, purée in a blender until smooth. Pour soup into a large saucepan and bring to simmer over medium heat. Add oysters and red pepper flakes and cook until oysters are just cooked through, about 2 minutes; stir in 1/2 cup cream.
Serve soup drizzled with more cream and topped with finely chopped watercress.