Pre-heat oven to 350 degrees. Line an 8x8 inch baking dish with parchment paper and set aside.
Whisk cornstarch into warm water and set aside.
Make the filling: Add rhubarb, strawberries, sugar, and lemon zest to a pot and bring to a simmer over medium heat. Simmer for about 10 minutes or until strawberries and rhubarb have begun to break down, then bring to a quick boil for 1 minute and remove from heat. Whisk in cornstarch slurry and set aside to cool and thicken.
Make the crust: Stir applesauce, melted peanut butter, light brown sugar, beaten egg and yolk, vanilla, and salt together in a medium bowl. Add rolled oats, protein powder, and cinnamon and stir until everything is combined. Press the mixture evenly into the prepared baking pan.
Pour strawberry rhubarb mixture over the crust and spread evenly. Set aside.
Make the topping: Cut in the chilled butter into the flour with a pastry cutter or food processor (or even with your hands) until the mixture resembles coarse crumbs. Mix in the oats, brown sugar, and cinnamon. Sprinkle the crumble over the filling.
Bake for 35-40 minutes or until the top is golden brown.