Peel cactus pears. If using plums, peel and remove pits. Place prepared cactus pears or plums in a food processor bowl or blender container. Cover and process or blend until smooth. Press cactus pear mixture through a sieve, reserving pulp and juice; discard seeds. If using plums, do not sieve. (You should have about 1 cup cactus pear or plum puree.)
In a medium saucepan combine cactus pear pulp and juice or plum puree with 3/4 cup of the strawberries, cut-up plums, sugar, and water. Bring just to boiling, stirring often.
Reduce heat. Cook fruit mixture, uncovered, on low heat for 30 to 45 minutes or until liquid is reduced by about one-fourth, adding the black pepper during the last 5 minutes of cooking, if desired.
Refrigerate mixture for 1 hour or until chilled. Turn mixture into a 9x9x2-inch pan. Cover and freeze until firm. Break frozen mixture into chunks; turn into a chilled mixing bowl. Beat mixture with an electric mixer on low speed until slightly slushy, then on medium-high speed until smooth.
Fold in remaining strawberries. Return to pan. Cover and freeze until firm. Meanwhile, chill desired serving bowls. Scoop and serve several small scoop of sorbet into each chilled serving bowl; garnish with cactus pear wedges, if desired. Makes 2 cups or 6 servings.