In a medium saucepan over medium-low heat, combine cranberry sauce, crushed pineapple, and orange zest. Stir the mixture well to combine.
Simmer, stirring often to prevent sticking, until the sauce has slightly thickened. This should take about 15 to 20 minutes.
Remove from heat. Serve warm garnished with rosemary or let the sauce cool completely. It will continue to thicken as it cools.
Store any leftovers in an airtight container in the refrigerator.