Add all of the ingredients to a small pot stir until the sugar is dissolved. Bring to a boil then reduce the heat to low and let it simmer for 10-15 minutes. Remove from the heat and store in a glass jar in the refrigerator.
For the cocktail
Pour the tequila and grapefruit syrup into a champagne flute and top with the cava. Garnish with the rosemary and enjoy!