In a medium bowl, whisk together the cornstarch, salt, and sweetener. Stir this mixture into the simmering berries.
Cook the berries over medium heat, stirring frequently at first and constantly toward the end, until the filling is very thick; you should be able to see the bottom of the pot when you draw a spatula through the berries.
Remove the simmering filling from the heat and stir in the cinnamon, butter, and lemon juice.
Pour the cooked berries over the fresh berries in the bowl, stirring to thoroughly combine.
Spoon the hot filling into the prebaked einkorn pie crust. Arrange the prebaked cutouts atop the filling, using as many or few as you like; any extras make delicious snacks! Let the pie cool fully, then chill until ready to serve.