Position a rack in the center of the oven and preheat it, then place a baking pan or oven-safe skillet on a parchment-lined baking sheet to catch any bubbling fruit juices.
In a medium bowl, whisk together the brown sugar, cornstarch, and salt, then add the rhubarb and strawberries and toss gently until the fruit is evenly coated.
Spread the fruit mixture evenly into the prepared baking pan and set it aside while preparing the topping.
In another bowl, whisk together the granulated sugar, oat flour, sweet rice flour, oats, baking powder, cinnamon, and salt until well combined.
Add the beaten egg and mix the topping thoroughly with your hands, squeezing and pinching the mixture until moist clusters begin to form.
Sprinkle the topping evenly over the fruit filling, covering the surface without pressing it down too firmly.
Drizzle the melted butter evenly across the topping and finish with a sprinkle of coarse sugar for extra crunch.
Bake until the topping becomes deeply golden and the fruit filling bubbles thickly around the edges.
Allow the crisp to cool slightly before serving warm or at room temperature.