Preheat the oven to 350 F / 180 C.
In a mixing bowl, melt the butter.
Add the flour, oats, salt, and brown sugar to the bowl and mix until combined.
In a greased 9x13 baking pan, add just under half of the crumble.
Slice the rhubarb stalks and add them to the 9x13.
Slice the strawberries and add them to the 9x13.
In a small saucepan, combine the water, white sugar, and cornstarch. Bring to a boil, stirring continuously, until the sauce has thickened.
Evenly add the sauce on top of the fruit.
Top with the remaining crumble and bake at 350 F / 180 C for 40 minutes.
Enjoy!
