Strain into a martini glass. Garnish with an orange slice and serve immediately.
Notes
Blood orange: You'll need at least one orange per martini for the juice. A hand juicer is helpful. Reserve one whole orange to cut into thin slices for a garnish.
Make ahead tips: Juice the oranges and lime; store separately in fridge. Slice an orange for garnishes. Chill the martini glasses. Make sure you have fresh ice.
Possible substitutions: If you like, replace the gin with vodka. Navel oranges or grapefruit could be substituted for the blood oranges.