1) In a saucepan, mix together cornstarch and sugar.
2) Slowly add in the water, while stirring.
3) Once the mixture is smooth without any lumps, add in 1/2 cup of blueberries.
4) Cook the mixture over low flame for 5 minutes, till it starts to thicken.
5) When the mixture becomes clear, add in lemon juice.
6) Take the saucepan off the flame and allow the mixture to cool down.
7) Take the cake or pie and top it with the remaining blueberries.
8) Spread the blueberry glaze on top to form a smooth layer.
9) Allow the cake or pie to chill, till the glaze sets.