Make the fennel simple syrup. Add the fennel seeds, sugar and water to a small saucepan. Bring to a boil, then reduce heat to medium and simmer for 5 minutes, stirring occasionally until sugar is fully dissolved.

Once sugar is dissolved and syrup is slightly thickened, turn off heat and allow to cool to room temperature. Strain into a jar or airtight container and store in the refrigerator until ready to use. Makes about 1.5 cups, and keeps up to 2 weeks in refrigerator.

Make the cocktail. Add simple syrup, vodka and grapefruit juice to a cocktail shaker or mason jar and shake well. Fill a highball glass with ice and pour mixture over. Top with club soda or sparkling mineral water and garnish with grapefruit slice. Make 1.