1. Preheat the oven to 375 F.
2. Combine the cinnamon, salt and flour (whisk these ingredients together). Then add the brown sugar and mix well. Using the food processor, or two forks or your fingers, cut in the butter until it is in very small pieces.
3. Mix in the oats, and set aside.
4. In a separate bowl, rub the granulated sugar together with the lemon zest. Mix in the cornstarch and vanilla, then add the blueberries. Stir to coat the berries.
5. Pour the berries into a large cast-iron skillet (or buttered 8x8 dish), and sprinkle the oat topping over the berries. Bake until the fruit is bubbling and the topping has browned - 50 minutes to an hour.
6. Serve with homemade whip cream or ice cream