2 oz gin 

1 oz fresh blood orange juice

1 Tablespoon orange simple syrup

club soda

Combine the gin, orange juice, and sugar in a shaker half-filled with ice cubes. Shake well and strain into a tall glass. Top off with club soda and garnish with an orange slice.

Orange simple syrup

½ cup water

¼ cup sugar

juice of one blood orange

Combine all ingredients in a small sauce pan over medium heat.  Heat until sugar is dissolved.  Remove from heat and let cool.

Variation:

Trade the gin with some bourbon, a few dashes of bitters, large orange peel and a big ice cube. Serve in a short glass.