Preheat your oven to 350F.
Mix the oat flour, oats, baking soda, salt, and cinnamon together in a large mixing bowl. Whisk together the banana, almond milk, coconut oil, maple syrup, and vanilla. Add the wet ingredients to the dry ingredients and mix well to combine. The mixture should be thick but easy to mix with a spoon or whisk. If it’s too sticky, add an extra few tablespoons of almond milk. Transfer this mixture to an oiled 8x8 or 9x9 square baking dish.
Toss the strawberries, rhubarb, sugar, and tapioca starch together and mix well. Spread this mixture over the base layer in the baking dish. Place the baking dish in the oven and bake for 15 minutes.
Place the oats, almond flour, brown sugar, salt, and melted coconut oil together in a food processor fitted with the S blade. Pulse until the mixture has formed nice, big crumbs. Remove the baking dish from your oven and sprinkle this mixture over the rhubarb and strawberry layer (which should be releasing some liquid by now, and even bubbling at the edges). Bake for another 20-25 minutes, or until the top is brown and the strawberry rhubarb layer has thickened up. Place the baking dish on a wire rack to cool completely. Cut into 9 or 12 squares, and serve.