• Preheat oven to 375F (190C) and lightly grease a 9x9” (23 x 23cm) square baking dish with butter. Set aside.
• In a large mixing bowl, combine berries, lemon juice, sugar, and salt and stir well.
  7 cups fresh or frozen mixed berries, 1 Tablespoon lemon juice, Pinch of table salt, ¼ cup granulated sugar
• Sprinkle flour over the berries and use a spatula to gently stir to coat the entire mixture. Set aside and prepare the crisp topping.
  3 Tablespoons all-purpose flour

Crisp topping

• In a separate large mixing bowl, whisk together flour, oats, sugar, baking soda, and salt.
  ¾ cup + 2 Tablespoons all-purpose flour, 1 cup old fashioned oats, ⅓ cup granulated sugar, ½ teaspoon baking soda, ¼ teaspoon table salt
• Use a pastry cutter to cut in the cold butter until the butter is well distributed through the mixture. This mixture will be very dry, once butter is in small pieces and mostly incorporated you can use your hands to pinch the dough together and create larger clumps if desired (be brief with your hands so you don’t warm the butter too much). When done the mixture should resemble coarse crumbs.
  ½ cup unsalted butter

Assemble and bake

• Use a spatula to give the berry mixture a brief stir then pour evenly into prepared baking dish.
• Sprinkle streusel mixture evenly over the berry layer.
• Transfer to 375F (190C) preheated oven and bake, uncovered, for 45 minutes or until topping is lightly golden brown and mixture is bubbly.
• Allow to cool for at least 10-15 minutes in the pan. Serve warm, topped with vanilla ice cream if desired.