In a large pot, whisk together the brown sugar, cane sugar, apple pie spice, and salt. Add the apple slices and lemon juice and toss until the apples are evenly coated in the sugar and spices. Set aside for 30 minutes to draw some of the juices out of the apples.

In a small bowl, stir together the cornstarch and water until smooth.

Set the pot with the apples over medium-low heat. Cook, stirring, until the apples soften slightly, about 5 minutes*. Raise the heat to medium and pour the cornstarch mixture over the apples. Cook, stirring, until the apple juices bubble and thicken, about 30 seconds. Remove from the heat and stir in the lemon zest and vanilla.

Allow the filling to cool completely before using in a pie, or serve warm with vanilla ice cream.