3 navel oranges
2 juice oranges (such as Valencia)
1 envelope unflavored gelatin
1½ ounces vodka (optional)
Cut all 5 oranges in half crosswise, then carefully juice them. Do not crush the skins of the navel oranges. Strain the juice through a mesh colander, then measure out 1¾ cups.
Carefully scrape out and discard the pulp from the navel orange halves to form 6 half shells. Set aside.
In a small saucepan, combine ¼ cup of the juice with the gelatin. Set aside for 5 minutes to let the gelatin dissolve. Heat the mixture over low heat, stirring occasionally, until the gelatin is dissolved and the mixture is clear, 3 to 4 minutes. Whisk in the remaining juice and the vodka, if using. Transfer the liquid to a measuring cup or small pitcher to make it easy to pour.
Arrange the orange shells, cut side up, in muffin tins or ramekins (to keep them upright). Divide the mixture between the shells, filling them halfway. Set the muffin tin in the refrigerator, then finish filling the oranges. Depending on the size of your oranges, you may only be able to fill 5 shells. Cover the filled shells with plastic wrap and chill until firm, at least 4 hours and preferably overnight. Cut each half into 3 wedges before serving.
Nutrition values per serving (not including vodka): 35 calories, 0 fat, 7 g carbohydrates, 0 fiber, 2 g protein, 0 cholesterol, 0 sodium.