
Preheat the oven to 375°F.
In a large bowl, stir together the sugar and cornstarch. Add the blueberries and lemon juice and toss well to combine. Transfer the filling to a greased 2-quart or 8-inch square baking dish.
In a medium bowl, stir together the flour, brown sugar, oats, butter, and cinnamon until crumbly but all of the flour is moistened by the butter. Squeeze the mixture into larger clumps using your hands. Scatter the topping evenly over the filling.
Bake for 50 minutes to 1 hour, or until the topping is golden brown and and the filling is bubbly. (Loosely cover with foil after 25 minutes if needed to prevent excess browning.) Transfer to a wire rack to cool for 30 minutes. Serve the crisp with vanilla ice cream, if desired.