Place the blueberries in a shallow 1 1/2 quart baking dish. Sprinkle on the sugar and flour, and toss to coat the berries evenly.
3
To make the topping, mix together the flour, oats, sugar, and cinnamon in a medium-size bowl. Drop in the butter cut into bits, toss, then rub them into the flour mixture with your fingertips until a texture resembling coarse crumbs forms. Sprinkle this all over the berries.
4
Bake 40-45 minutes for fresh berries, 45-50 minutes if using frozen berries. When done, the crisp will be golden brown on top and bubbly on the sides. Let cool on a wire rack to room temperature. Serve small portions with a scoop of vanilla ice cream, or a dollop of whipped cream, if desired.
Tip: For the juices to thicken it is essential that they bubble (or boil) under the blanket of crumbs.