Place chicken stock or broth and butter into a large saucepan and bring to a boil. Add the polenta and cook, stirring constantly, for 15 minutes. Add remaining ingredients; cook and stir for 5 more minutes. Check the seasoning, then add salt & pepper to taste.
Pour equal amounts of mixture into two 8" square cake pans and chill in the refrigerator for at least 1 hour. Unmold polenta from pans and slice into 1/2" thick slices. Heat slices in a 400 degree oven on a cookie sheet for 10 minutes before serving.