Preheat the oven to 350 degrees F. Spray a 1 1/2-quart square baking dish (8- by 8-inch) with nonstick spray.
For the filling: Combine the berries, granulated sugar, flour, lemon juice and vanilla into a bowl and mix well. Add to the prepared baking dish.
For the topping: Add the oats, brown sugar, pistachios, flour and butter in a medium bowl and mix all together with your hands. Sprinkle the mixture evenly all over the berries and pop in the oven and bake until the berries are soft, the filling is bubbling and the crisp topping is lightly golden, about 30 minutes. Let cool for 30 minutes so the filling can thicken up. Serve with a big ol' scoop of vanilla ice cream or whipped cream on top.