A capon, which weighs a good eight or nine pounds, is a young male chicken that has been neutered. The snow-white breast meat is very tender and finely textured. You can find capons fresh or frozen at butcher shops and many supermarkets.
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.