Preheat the oven to 375°F. Butter an 8x8-inch or similar size baking dish. Set aside.
In a large bowl, toss the blueberries granulated sugar, cornstarch, and lemon juice together until well combined. Transfer to the prepared baking dish.
⅓ cup granulated sugar, 1 tbsp cornstarch, 5 cups fresh or frozen blueberries, 2 tsp lemon juice
To a medium bowl, add in the butter, brown sugar, flour, salt, cinnamon, and oats. Use your hands to mash and combine the ingredients until pea-sized bits form.
½ cup unsalted cold butter, ½ cup brown sugar, 1 cup all-purpose flour, ¼ tsp kosher salt, ½ tsp cinnamon, ¼ cup quick oats
Sprinkle the crisp topping over the blueberry filling.
Bake for 35-40 minutes or until the fruit is bubbling around the edges and the topping is golden brown. Let cool for 5-10 minutes before serving.