1. Stir gelatin into cold water until dissolved. Add boiling water, orange juice concentrate and lemon juice. Stir until smooth. Add sugar and continue stirring until dissolved. Refrigerate until mixture starts to thicken, about 1 hour.
2. Whip heavy cream to stiff peaks. Fold into orange juice mixture.
3. Tear cake into bite-size pieces and place in a 13 x 9 x 2-inch baking dish. Pour orange juicewhipped cream mixture evenly over cake pieces. Cover and refrigerate overnight.
4. Scatter nuts and orange zest, if desired, over the top and serve.