Preheat Oven: Preheat your oven to 375°F (190°C) and line a 9×9-inch baking dish with parchment paper.
Prepare the Berry Filling: Measure out the frozen berries. If using large strawberries, cut them into smaller pieces. In a small saucepan over low-medium heat, combine the berries, lemon juice, coconut sugar, maple syrup, and a pinch of sea salt. Cook for 5-10 minutes, stirring occasionally, until the berries release their juices and become slightly soupy.
Thicken the Filling: Remove 1-2 tablespoons of the berry liquid and mix it with the tapioca starch in a small cup until smooth. Pour this mixture back into the saucepan and stir continuously for about 1 minute, until the filling thickens. Remove from heat and set aside.
Make the Oatmeal Crust and Topping: In a large mixing bowl, combine the rolled oats, almond flour, baking soda, and sea salt. Add the maple syrup, vanilla extract, and solid coconut oil. Use a fork to mix the ingredients, cutting in the coconut oil until well incorporated. You may need to use your hands to fully combine the mixture.
Assemble the Bars: Reserve ½ to ¾ cup of the oatmeal mixture for the crumble topping. Press the remaining oatmeal mixture evenly into the prepared baking dish to form the crust.
Add the Berry Layer: Spread the berry mixture evenly over the oatmeal crust.
Add the Crumble Topping: Sprinkle the reserved oatmeal mixture over the berry layer, using your fingers to create small clumps for a textured topping.
Bake: Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the edges turn lightly golden.
Cool and Serve: Let the bars cool completely in the baking dish before slicing to ensure they hold together. Store the bars lightly covered in the refrigerator for a healthy breakfast throughout the week.