James Beard's Roquefort-Filled Mushrooms

8 oz (250 g) Roquefort or other good quality bleu cheese
1/2 cup (125 ml) butter, softened
1 tsp (5 ml) dried mustard
24 medium mushroom caps
1/2 cup (125 ml) finely chopped green olives

Combine the cheese, butter, and mustard, mixing until smooth. Pipe or spoon the cheese mixture into the mushroom caps and sprinkle with
chopped olives. Makes 24 to serve 6 to 8 as an hors d'oeuvre.