Combine cornstarch, sugar, egg yolks, fruit puree, and salt in a mixing bowl.
Heat milk and vanilla until just boiling. Pour small amount of hot milk into egg mixture while whisking to prevent scrambled eggs. Pour egg mixture back into milk pan, again whisking to prevent curdling. When custard starts to thicken, add butter and remove from heat. Whisk until smooth; chill and reserve.
Fill all eclair shells with custard cream.
Top each filled shell with layer of more custard cream. Arrange chopped cucumbers and honeydew melons. Garnish with chervil and edible petals.