Combine 3 cups of the blueberries, butter and lemon juice in a non-reactive saucepan over medium heat. Cook until the berries release their juices.
Stir together the sugar, corn starch and salt, add to the berries. Return to a simmer then stir continuously till the mixture thickens and becomes glossy, about 5 minutes.
Remove from the heat, pour the mixture in a bowl and cool for about an hour.
Once cooled, fold the remaining blueberries, strawberries and lemon zest into the cooked mixture and gently pour into the baked shell.
