Add all the ingredients for the syrup in a small pot and bring to the boil. Reduce the heat and let it simmer for 10 minutes.
Remove from the heat and let the syrup steep and cool for about one hour.
Store it in a glass bottle or glass jar and keep refrigerates for up to one week. → If you don't like grapefruit zest in your drink, pour it through a fine-mesh sieve just before serving.
Grapefruit Rosemary Mocktail
Add the syrup and then the fresh grapefruit juice in a cocktail glass.
Top it with sparkling water and ice cubes. Decorate with fresh rosemary or grapefruit slices.
Notes
Grapefruit: About 3 fresh pink grapefruits are enough to prepare the syrup and make 4 mocktails.
Syrup: Make the syrup a day ahead and store it in the refrigerator for even more flavor.
Syrup yield: One batch of syrup makes up to 20 mocktails, depending on how sweet you like them.
For kids: If grapefruit juice is too bitter, simply serve the syrup topped with sparkling water.
Larger servings: This recipe is designed for small cocktail or champagne glasses. Double the ingredients if using regular glasses.