12 servings
Source: Bon Appétit, November 2005
Cut 1 orange in half through stem. Cut each orang half crosswise into 6 slices; reserve for garnish. Using small knife, cut peel and white pith from remaining 2 oranges; chop oranges. Transfer chopped oranges with juices to bowl; mash with potato masher or wooden spoon. Stir in gin, sugar, and angostura bitters. Strain into 2-cup measuring cup, pressing on solids to release liquid. Chill 4 hours.
Pour scant 2 tbsp gin mixture into each of 12 glasses; fill with Champagne and garnish with orange.