Remove tough muscle from side of each scallop if necessary. Pat scallops dry and season with salt and pepper. In a 10-to 12-inch non-stick skillet heat oil over moderately high heat until hot but not smoking and sauté scallops 1 to 2 minutes on each side (depending on size), or until golden and just cooked through. Transfer scallops with tongs to a plate and keep warm, covered loosely.
In oil remaining in skillet cook shallot over moderate heat, stirring, until softened. Add wine and boil, scraping up brown bits, 1 minute. Stir in water and mustard and simmer until reduced to about 1/4 cup. Add butter and swirl skillet, returning skillet to heat as necessary, until butter is just incorporated into sauce. Season sauce with salt and pepper.
Spoon sauce onto a small platter or 2 plates. Top sauce with scallops and sprinkle with scallion. Serve scallops with caraway rice cakes.
