In a small saucepan, mix together vinegar, orange zest and juice, sugar, rosemary, salt and pepper. Bring mixture to a boil then reduce heat to low. Simmer mixture until thickened and reduced to 2/3 cup (150 mL), about 15 to 20 min.
Remove from heat, strain and cool to room temperature. Transfer to a small jar or bottle and keep refrigerated up to 1 week. Let stand at room temperature for 15 minutes before serving to maximize flavour.