Line an 8-by-4-inch loaf pan with plastic wrap and spray the plastic wrap with nonstick cooking spray.
In a medium saucepan, combine the cranberries with the apple, sugar and water. Bring to a boil and cook over moderately high heat, stirring frequently until the cranberries are completely broken down and the sauce is very thick, about 15 minutes.
Scrape the cranberry sauce into the prepared pan and refrigerate until chilled, about 3 hours.
Invert the jelly onto a serving plate and remove the plastic wrap. Garnish with fresh cranberries and rosemary sprigs. Slice with a serrated knife before serving.