- 1 small leek (white and pale-green parts only), coarsely chopped
- 1 large shallot, coarsely chopped
- 1 garlic clove, coarsely chopped
- 1 large bunch watercress, coarsely chopped (about 4 cups packed)
- 1/4 cup (1/2 stick) unsalted butter
- Kosher salt and freshly ground black pepper
- 1/2 cup finely grated Gruyère
- 1/2 cup heavy cream
- 1/4 cup finely chopped flat-leaf parsley
- 24 large oysters, freshly shucked, on the half shell, with juices
- Pulse leek, shallot, and garlic in a food processor until finely chopped. Transfer to a small bowl; set aside. Pulse watercress in processor until finely chopped but not a paste. Transfer to a medium bowl; set aside.
- Melt butter in a large heavy pot over medium-low heat. Add leek mixture; season with salt and pepper and cook, stirring often, until translucent, 7-8 minutes. Increase heat to medium. Add watercress; cook, stirring often, until watercress is wilted and tender, 8-10 minutes. Stir in cheese, cream, and parsley. Season to taste with salt and pepper. Transfer to a medium bowl; chill until cold. DO AHEAD: Can be made 1 day ahead. Cover; keep chilled.
- Preheat broiler. Arrange oysters on half shells on a rimmed baking sheet. Dividing equally, spoon watercress mixture over oysters, spreading to cover completely.
- Broil until cheese is melted, top of watercress mixture begins to brown in spots, and oysters are just cooked through, 3-4 minutes. Serve immediately.