How to Make It
In a large skillet of salted boiling water, blanch the leeks just until fork-tender, 8 to 10 minutes. Transfer to an ice bath to cool; drain well and pat dry. Wipe out the skillet.
Meanwhile, in a saucepan, bring the wine to a simmer. Cook over moderately low heat until reduced by half, 10 minutes. Off the heat, whisk in the butter, a few cubes at a time, until smooth. Season with salt.
In another small saucepan, bring the milk to a boil. Add the cheese, cover and let stand off the heat for 3 minutes. Scrape the mixture into a blender and puree until smooth. Season with salt. Keep warm.
Return the leeks to the skillet and pour the Riesling butter over. Cook over moderately low heat, stirring, just until warmed through, 2 minutes. Spoon a little cheese sauce onto each of 4 plates. Top with the leeks and spoon some Riesling butter on top. Garnish with the watercress and pickled onions and serve.