Step 1

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.

Step 2

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.

Step 3

Remove from the heat, pour the mixture in a bowl and cool for about an hour.

Step 4

Once cooled, fold the remaining blueberries, strawberries and lemon zest into the cooked mixture and gently pour into the baked shell.

Step 5

Cool until set, at least 4 hours.

Step 6

Serve with whipped cream or ice cream if desired.