Preheat oven to 375°F, 185°C, or gas mark 5. Grease a 9 x 9 inch square baking pan with cooking oil spray and set aside.
If using frozen blueberries let them thaw in a colander. In a large mixing bowl, combine blueberries, lemon juice, lemon zest, cornstarch, and brown sugar. Toss gently to coat the blueberries. Pour the blueberries pan.
In another mixing bowl, combine softened butter, brown sugar, salt, and cardamom. Stir together with a rubber spatula until combined. Add the flour, walnuts, and oats and blend until just combined. Evenly distribute the topping on top of the blueberry layer.
Bake for 35 to 40 minutes until the top is lightly browned. Place on a wire cooling rack to cool down for 10 minutes. Can be served warm with ice cream or chilled with a little whipped cream and mint.