Combine the brown sugar, cinnamon, salt, and flour. Mix well to combine. Using a food processor, or two forks or your fingers, cut in the butter until it is in very small (pea-sized) pieces.
Mix in the oats, and set aside.
In a separate bowl, rub the granulated sugar together with the lemon zest. Mix in the cornstarch, then add the blueberries. Stir to coat the berries.
Pour the berries into a large cast-iron skillet (or a buttered 8x8 baking dish), and sprinkle the oat topping over the berries. Bake until the fruit is bubbling and the topping is browned — 50 minutes to 1 hour.
Serve with ice cream!
Recipe by The Kitchen Paper at http://c02.ddd.mwp.accessdomain.com/skillet-blueberry-crisp/