Place the pears in a saucepan and cook on low heat until they are mushy. If they are not very juicy, add a couple of tablespoons of water. Remove from heat and set aside.
Place the elderberries, water and lemon peel in a large saucepan and cook 25 minutes until the berries are falling apart. Strain through a sieve to get rid of the seeds and skins and place the liquid back in the saucepan.
Combine the cornflour with 3 tablespoons water and whisk into the hot soup. Let thicken, then season to taste with lemon juice and sugar, stirring well. Let cool then chill before serving.