INGREDIENTS
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1 small leek (white and pale-green parts only), coarsely chopped
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1 large shallot, coarsely chopped t
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1 garlic clove, coarsely chopped
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1 large bunch watercress, coarsely chopped (about 4 cups packed)
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1/4 cup (1/2 stick) unsalted butter
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Kosher salt and freshly ground black pepper
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1/2 cup finely grated Gruyère
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1/2 cup heavy cream
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1/4 cup finely chopped flat-leaf parsley
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24 large oysters, freshly shucked, on the half shell, with juices