Preheat the oven to 350℉ and line a 9x13” pan with parchment paper. Set aside.
In a large bowl, combine oats, brown sugar and flour until combined. Add the butter and mix with an electric mixer, a pastry cutter or your hands until combined.
Set aside 1 packed cup of the oatmeal crumble mixture. Press the rest into the bottom of the prepared pan, covering the bottom entirely. Bake bottom crust for 10 minutes until dry on top.
In a medium bowl, combine your strawberries, rhubarb, sugar and corn starch. Spread on top of crust and crumble the remaining oat mixture on top – it will not cover the top completely. Press down the oat mixture slightly.
Bake for 40-45 minutes until the filling is thickened and bubbly and thickened and the oats are light golden brown.
Cool to room temperature. I highly recommend chilling before cutting – they will set completely and cut more cleanly. Store in the refrigerator