Add 1 cup of blueberries, ⅓ cup sugar, lemon juice, and salt to a saucepan and cook over medium heat until blueberries burst and mixture is thickened.
Add remaining 1 cup of blueberries, apple, and lemon zest. Stir to combine. Spread into the pie plate.
Prepare Topping: Melt the butter in a small saucepan and keep cooking it, swirling the pan occasionally, for about 5 minutes or until it turns golden. Pour into a bowl. Stir ¾ cup of sugar into the butter, then the eggs, then the flour and cinnamon.
Pour over the fruit and sprinkle with raw sugar. Bake for 40-45 minutes, until golden and crusty, and the juices ooze from around the edges.
Serve warm with vanilla ice cream, whipped cream or thick vanilla yogurt.
Notes
*Instead of the blueberry filling you can substitute - 8 or so plums, thickly sliced, or apricots, or 3 peaches, or 3 tart apples, peeled and thinly sliced + 2 Tbsp. sugar (or to taste, according to the sweetness of the fruit)