The Cointreau livens up this typical lemon ice treat.
In a large bowl, mix together the lemon juice, lemon zest, orange juice, simple syrup, and Cointreau until well combined.
In the bowl of an electric mixer, whip the egg whites until stiff peaks form. Briskly whisk the egg whites into the lemon-juice mixture until well combined. Freeze in an ice-cream maker according to manufacturer's instructions. Store, frozen, in an airtight container up to 3 days.
The day before you make this sorbet, adjust your freezer to the maximum setting.