A paillard is simply a skinless, boneless chicken-breast half that you butterfly and pound thin. It's one of the speediest routes to a lean meal: While you prep and saute it, roast some squash and red onion, then place it on top with fresh spinach and crisp fried sage.
You can make the bechamel sauce three days ahead (let it cool completely before refrigerating). The onions can be cooked, peeled, and refrigerated overnight. Heat the onions and sauce together over low heat before serving.