Preheat the oven to 375 degrees F. Whisk the corn starch into the orange juice until dissolved.
Combine all the ingredients for the Strawberry - Rhubarb Filling in a large bowl, and toss well. Let sit while you make the Crisp Topping.
Combine all of the ingredients for the Crisp Topping, except the turbinado sugar, in the bowl of a food processor fitted with a blade. Pulse 10 - 12 times until the butter is the size of small peas. Alternatively you can combine all of the ingredients except the turbinado sugar in a bowl and use a pastry blender to cut the butter into the flour, brown sugar, oat mixture. Either way works great.
Pour the Filling into a baking dish and sprinkle the Crisp Topping evenly over its surface.
Sprinkle the turbinado sugar on top. This gives the topping a nice crunch.
Bake for 35 - 40 minutes, until the crisp topping is lightly browned and the filling is bubbling.
Let cool for 30 minutes before digging in.