Sauté chicken livers in butter about 5 minutes until cooked through. Remove from skillet and set aside. Sauté onion in same pan about 3 minutes until softened but not brown. Puree livers and onion in a food processor or blender. Add brandy, mustard, allspice and pepper. Puree or grind again until mixture is smooth. Spread in shallow pan, cover and refrigerate until thoroughly chilled. Soften remaining butter and beat until light and creamy; add liver mixture and mix well. Fold in finely chopped pistachios; salt to taste. Spoon into a crock or mold, cover and refrigerate until firm. Bring to room temperature to serve. makes about 3 1/2 cups.