Flatten bread with rolling pin; cut each slice into four pieces. Put butter in shallow dish; dip both sides of bread in butter; press into miniature muffin cups. Bake at 325 for 14 minutes or til golden brown. Remove form pans to wire racks to cool.
In large bowl, beat cream cheese, mayonnaise, sour cream and horseradish til blended. Just before serving, stir in shrimp and onions; spoon into cups. Garnish with dill if desired. Refrigerate leftovers. Makes 8 dozen