In a small pot, place lemon or lime juice. Gently sprinkle gelatin over top. Wait for about 2-3 minutes until gelatin dissolves. Bring the heat to medium. Very gently stir or shake the pot until the gelatin lumps are gone. Stir in granulated sugar and bring to a boil, stirring to dissolve sugar. When sugar has completely dissolved into a clear liquid, turn heat off, remove pan and let liquid cool for about 10-15 minutes. Add sour cream to the liquid and stir to completely incorporate. Spoon mixture into 4 dessert or parfait glasses. Cover and refrigerate overnight. When ready to serve, garnish with fresh citrus.