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.
A blue cheese bechamel makes this tart deliciously rich, so you need only a small piece. It's a good starter or side dish for a holiday meal, but it works just as well for lunch or a light dinner with a green salad.
A capon is a large bird bred to have more white meat; it may have to be special-ordered from your butcher. A seven- to eight-pound roasting chicken can be substituted. This recipe was originally developed for a Passover dinner; butter and flour may be substituted for the matzo meal and margarine. Soak the pearl onions in warm water for 15 minutes to make peeling easier.