Preheat oven to 350°F. Lightly coat a 9-inch pie plate or oven-safe skillet with butter or nonstick spray.
In a large bowl, add berries, then sprinkle lemon juice over the top. Combine granulated sugar and 2 tablespoons flour and sprinkle over berries and mix well. Pour berries into prepared pan.
In a small bowl, combine oats, pistachios, brown sugar, remaining 2 tablespoons flour, salt and cinnamon. Cut in butter using a pastry blender or two forks until large clumps are formed and no dry bits remain. Sprinkle topping evenly over fruit.
Bake for 45-50 minutes, until browned and bubbly. Serves 6.