Preheat the oven to 350ºF (180ºC) and lightly grease an 8-inch square or oval baking dish with butter. Set aside.
Stir the blueberries, lemon juice, and sugar in a mixing bowl. Add the tapioca starch and stir well to combine. Pour into the prepared baking dish.
In a second bowl, stir the oats, brown sugar, and flour. Rub the butter into the mixture using a fork or your fingertips. Once the flour is incorporated, scatter the mixture over the berries. Bake the crisp for 50 to 55 minutes or until the top is golden and the blueberries are bubbling. Cool on a wire rack for 20 minutes before serving warm with ice cream or whipped cream.
Notes
Use cold butter for the topping. This helps create a crumbly texture that crisps in the oven. If your butter is at room temperature, place the oat topping in the fridge for 20 minutes to firm up before baking.
Storage
Refrigerating: Store any leftovers in an airtight container in the refrigerator for up to 3 days. Reheat in the oven at 350°F (175°C) until warmed through.
Freezing: You can freeze the baked crisp for up to 3 months. Cover it tightly with aluminum foil or place it in a freezer-safe container. Thaw overnight in the refrigerator and reheat in the oven to preserve the texture of the topping.
Reheating Tips: To avoid a soggy topping when reheating, place the crisp back in the oven at 350°F (175°C) for about 10-15 minutes or until the topping is crisp again and the filling is bubbly. Avoid microwaving, as it can make the topping soft.