1. Combine plums and sugar in a large bowl; toss well to coat. Let mixture stand at room temperature for 1 hour.
2. Place plum mixture in a food processor or blender; process until smooth. Press mixture through a fine sieve over a large bowl; discard solids. Add crème fraîche, stirring with a whisk until blended. Cover and chill. Pour mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon ice cream into a freezer-safe container; cover ice cream, and freeze 1 hour or until firm.