Preheat oven to 350F.

Make the crust: In the bowl of a food processor, add the flour, sugar, salt and cubed butter. Pulse until the mixture begins to turn crumbly. While the food processor is running, add the apple cider vinegar and then stop the machine.

Turn the dough out into a 9x13 baking pan. Use two forks to press the dough evenly into the pan. Place in freezer for 15 minutes while you make your remaining components.

In a large bowl, toss together the blueberries, lemon juice, lemon zest, sugar and cornstarch.

In your food processor bowl, pulse together the flour and sugar for the topping. Add the softened butter and pulse until crumbles form.

Remove your baking pan from the freezer. Top with the blueberries, then sprinkle the crumb topping over.

Bake in preheated oven for 45 minutes, until topping is browned and filling is bubbling. Let cool before cutting into squares and serving.